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While the title seems to already anwser the question on it's own (there's no way you can detect something undetectable), should that really be the case?

Let's check three examples:

  • Collapsing Floor trap: it looks identical to all the normal floor around it, except putting too much weight on it causes it to collapse.
  • Door Trap: a mechanism that triggers when a heavy door is opened, causing a massive explosion that blasts everything within 100 feet to smithereens, possibly launching the door at the party at high speeds.
  • Magic Trap: triggered by an Alarm spell, except the material components were eschewed / made invisible, as well as applying a nondetection spell on it to floor detect magic and find traps spells,

Let's say we have a Rogue with 30 Passive Perception, as well as expertise in Arcana and whatever other skills might be useful.

Do they notice the supposedly undetectable floor trap by noticing the floor sag by a tiiiiny bit, or feeling that is starts to give way as soon as they lightly step onto it, quickly withdrawing their foot before it triggers?

Do they notice that the door's movement isn't quite right - actually, they would have no idea how heavy the door is, and as soon as they hear the mehcanism's clicking, it's already too late. Does perception translate into intuition/gut feelin/sixth sense in this situation, or is this trap outright impossible to prevent, not even allowing perception checks or whatnot, making the trapfinder rogue feel useless?

As for the magic trap, is the only way to detect it to have Truesight (not happening before level 20), or even blindsense (still very high level for a relatively mid-to-low-level trap) to somehow feel the invisible arcane runes/silver string - that sounds like a huge stretch.

TL;DR Can a Rogue detect a trap that is physically and magically impossible to notice, if their perception (and arcana) are high enought? Or are some of the more clever traps just impossible to find, making them top choices against intruders in the D&D multiverse?

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  • \$\begingroup\$ With that definition of a "Collapsing Floor trap", every floor is such a trap because every floor gives in beyond a certain weight/force. \$\endgroup\$ – David Foerster Jun 12 at 15:56
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    \$\begingroup\$ Maybe you should ask, "Can traps be undetectable in D&D 5e?" or "When should I use undetectable traps in D&D 5e" or even "What DC should an underectable trap be in 5e?" \$\endgroup\$ – WakiNadiVellir Jun 12 at 19:49
  • \$\begingroup\$ Other ways to notice the door trap: it's aged enough to have been there for a while, but lack of wear on the door handle and nearby floor suggest that it's never been used. Why does this door have a turning handle when a knob would have done the job and been simpler to make? There is a smell of saltpeter. The design looks very similar to other doors in this building but is subtly different, suggesting that it was made by a different person who didn't want it to be noticed as different ("only the Gadget Dwarves of Zomg use those hinges!") etc. etc. \$\endgroup\$ – Geoffrey Brent Jun 13 at 0:02
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If something is impossible, it is impossible

I'll quote the opening pages of the Player's Handbook.

In cases where the outcome of an action is uncertain, the Dungeons & Dragons game relies on rolls of a 20-sided die, a d20, to determine success or failure.

If the outcome of an action is certain, then there is no roll or ability check to make to attempt to change the outcome.

Rogues have no special trap-finding sense

While Rogues are traditionally good trap-finders, and they can definitely optimise Perception and Investigation skills to be ludicrously high, rogues have no special features which let them supernaturally detect traps (by the book, anyway). There may be features or spells from other parts of the game which can help detect traps, but not from the rogue class features.

It is up to DM judgement as to whether a ludicrously high result on a Perception check results in nigh-supernatural senses. Fifth edition defines a DC 30 ability check as being typical of a "nearly impossible" task (PHB p.174). While previous editions of the game may have regularly had bonuses and DCs far in excess of this at higher tiers of play, fifth edition follows bounded accuracy, meaning there is little point in defining DCs much higher than 30 because a designer should not expect a regular character to easily exceed that DC (although players cheesing the system might be cause to reconsider). What counts as "nearly impossible" as opposed to just plain impossible is up to the DM and the tone and genre they want at their table. But in general the DM has no reason to set a finite DC for an impossible task - they just declare it impossible, no roll required.

But is the trap really undetectable?

In your cases, you need to consider whether all components of the trap are completely undetectable. In some cases this may be true, such as in the heavy door scenario or the magical alarm. But in some cases the disguise may not be perfect.

To take the collapsing floor as an example, there might be hairline fractures in the floor, or it might sag slightly, or when a single foot or 10-foot pole is placed on it the floor sounds hollow or shifts slightly. These might be very difficult signs to detect, but someone with an astonishingly high Perception should notice them.

Even in the door scenario, it may be possible to notice a glint of the mechanism through a gap around the edge of the door. Trap makers aren't perfect.

But even if the mechanism of the trap is somehow perfectly obscured, traps do not exist in isolation. Traps will be avoided by the denizens of the dungeon. As such, there should be evidence that the space associated with the trap is unused while nearby spaces are used. The trapped door, on closer inspection, shows no sign of use. The collapsing floor has dust but no footprints. And so on.

Checking for lack of footprints applies to old traps. For very recently created traps, you might instead find evidence that the trap-maker had been about recently. This is perhaps less of a tell-tale sign of a trap, but it's something.

With a very high Perception, you need to think very carefully about whether there is truly absolutely no possible way to detect even a clue of the trap's presence, even indirectly.

If you are the DM, you also need to consider why you are designing a literally undetectable trap. If it's a matter of "that's what the NPCs would do", then consider also the possibility than the NPCs might not be flawless trap makers able to remove every single clue of a trap's existence, and that other factors may indirectly reveal the trap anyway. It may help to consider whether the PCs could make such an undetectable trap, or whether there would be some limitation on what the PCs could plausibly achieve.

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    \$\begingroup\$ @tomasz D&D 5e strongly encourages the DM to simply not allow a roll for attempting something impossible, and also has far fewer ways to gain bonuses to most checks. Something like DC 40 might be barely within reach for a heavily optimized character with a lot of support, but assigning unreasonable difficulties like 80+ is not really productive. \$\endgroup\$ – Kamil Drakari Jun 10 at 18:35
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    \$\begingroup\$ It's worth noting that DC 80 in 3.5e and 5e are very different beasts. 3.5e had the expectation that high level characters would be kitted out with a magic arsenal, and there were a lot more places where bonuses could stack. 5e reigned that in a bunch, so it wouldn't shock me if a DC 80 is technically possible in 3.5e with enough magic and character specialization. \$\endgroup\$ – Oscar Smith Jun 11 at 3:02
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    \$\begingroup\$ Re: that last point: Also bear in mind the goose vs gander problem - the PCs can (and will) pull any trick you let the NPCs get away with. Consider any factors you’d factor in if a PC had set the trap and you were sending an NPC rogue at it \$\endgroup\$ – Pingcode Jun 11 at 6:30
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    \$\begingroup\$ The last paragraph is probably the most important in my opinion. Why design an undetectable trap? The only reason to make a trap that can't be detected is to ensure that it is triggered. If you want the players to find it, make it detectable. \$\endgroup\$ – Carson Jun 11 at 13:55
  • \$\begingroup\$ Another point about to consider about traps as part of world-building: did the designer have a plan for them to be resetable (manually)? If a collapsing floor makes the secret lair unusable for the BBEG, s/he would have to think twice before having one installed in the first place, vs. other means of deterring interlopers. Also, don't build your hideout in a way that destroys itself if the new guard puts a foot wrong. \$\endgroup\$ – Peter Cordes Jun 11 at 15:29
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In addition to BBeast's answers, consider the context of how they might use senses other than sight to detect traps.

Collapsing Floor trap

It's fairly easy to hear when a wall is hollow. A skilled thief could easily hear that there was a cavity under a floor. Different ores and materials smell different. The floor board which is made to break easily may smell different because of that. Concealing these things is gonna be immensely expensive and difficult.

Door trap

Humans can develop some degree of echolocation. Are you sure that whatever bomb on the other side of the door is gonna sound the same as a normal door? Humans can certainly smell explosive powder- it has a very distinctive scent. Humans can know door models. Are you sure they don't know how this door is supposed to feel when they open it?

Magical trap

Arcana (intelligence) can detect these regardless of non detection spells. Using your senses isn't divination magic. Casting a bunch of spells on it isn't gonna make it less visible to Arcana senses any more than setting things on fire makes them less visible to eyes.

Any character can attempt an Intelligence (Arcana) check to detect or disarm a magic trap, in addition to any other checks noted in the trap's description. (DMG, p. 121)

There's no special exemption that makes invisible less vulnerable to this. Maybe there's a mystical hum, or a smell of sulfur.

So, always consider what means other than just seeing they might use.

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  • \$\begingroup\$ I'm not sure the clause about magical traps being detectable with an arcana check is meant to apply to all magical traps, or just the magical traps listed below in the same section. The very next sentence says "The DCs are the same regardless of the check used," which strongly implies that it is referring only to the traps listed below, with DCs listed for detection. Alarm has no DC listed for detection of its presence. \$\endgroup\$ – Ryan C. Thompson Jun 11 at 13:22
  • \$\begingroup\$ The DC is for the effect, not the triggering mechanism. The triggering section makes clear there can be many triggers. \$\endgroup\$ – Nepene Nep Jun 11 at 14:07
  • \$\begingroup\$ I'm talking about the DCs for spotting the traps, not the DCs associated with the traps' effects, since that's what this question is asking about. E.g. for the fire-breathing statue: "The DC is 15 to spot the pressure plate, as well as faint scorch marks on the floor and walls." Since it is a magic trap, it can also be spotted with a DC 15 Arcana check. Regardless, an Alarm spell has no DC for anything. \$\endgroup\$ – Ryan C. Thompson Jun 11 at 16:09
  • \$\begingroup\$ @RyanC.Thompson: Though a wholly reasonable ruling (not based on any specific rules I know of, but the obvious choice) would be to use the caster's spell save DC. That said, Alarm isn't a "trap" to my mind, so allowing Arcana users to detect it through searching risks stepping on the toes of classes with spells and abilities to detect magic. If the mechanism that harms them is also a statically triggered spell, I'd allow detection via Arcana, but disarming would depend on whether there are physical components they can alter to disrupt the trap vs. a magical field that only spells can touch. \$\endgroup\$ – ShadowRanger Jun 11 at 18:03
  • \$\begingroup\$ To detect it, you make a roll against the trap's DC. The alarm spell being the trigger doesn't change the DC. If the alarm spell is the trigger for the trap, it is detectable as part of the trap. If the trap detects the triggering of an alarm spell somehow, you can detect whatever detects it. \$\endgroup\$ – Nepene Nep Jun 11 at 19:55
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I want to side-step your question about game mechanics and instead ask whether an undetectable trap should even exist inside your game. Thus, I defer to the following question:

What narrative purpose would an undetectable trap serve?

Option 1: The DM intends to spring the trap no matter what once it is triggered.

Sometimes it makes sense to remove some player agency to advance a narrative that hopefully rewards their lack of agency, e. g. to kick off an interesting adventure or to hasten a group of bumbling PCs who missed all previous adventure hooks to the Good Part™.

In such cases, it may make sense to skip the part where PCs may have a chance to detect the trap and have them directly fall into it (also see Nepene Nep's argument on the pointlessness of rolls without chance of success). Although I'd argue that the very discovery of the trap can be an equally good kick-off event because it sparks curiosity: Why is there a trap? Who put it there? What was their goal? What/Whom are/were they targeting or guarding?

Option 2: The DM does not intend to spring the trap no matter what.

If that's not the intent, why make the trap impossible to detect to PCs?

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  • \$\begingroup\$ You mean you don't place TNT blocks under redstone ores for the lulz? \$\endgroup\$ – Joshua Jun 13 at 2:51
  • \$\begingroup\$ I knew this question would pop up - while the point is very valid, i needed an anwser for a kind of special campaign setting that i don't really intend to be use in actual play, but rather serve as a basis for something else - like a webcomic or novel. \$\endgroup\$ – Keinname Jun 16 at 6:33

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