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I thought I knew, at first, how passive perception versus hidden/invisible creatures and/or well laid traps worked. I have a group of level 2 adventurers that are about to come upon their first trap after a few encounters where a few Wolf Spiders were able to easily sneak up on the group due to their +7 Stealth and no one being perceptive enough to spot them, and no one making any perception checks to see if anything else was sneaking around.

Enter Quist, my new join who has no prior information on what unfolded so far in the campaign. Quist is a Human Variant Rogue with a Wisdom of 14 (+2) , proficiency with Perception (+2), the Observant Feat: (+5 Bonus to Passive Investigation AND Perception), and let's not forget the Rogue's Expertise ability: double your proficiency bonus when making checks with two chosen proficient abilities or thieves tools.

So let's add that up. The math is 10 plus any bonuses to figure out Perception. 10 + 2 (wis) + 5 (Observant) + 4 Proficiency Bonus (Expert Perception doubles Proficiency bonus) = 21

So this Rogue is smashing through a DC check of 20 or lower against Stealth, Hide, Traps and the like PASSIVELY. What difficulty level is a DC 20 check? Hard. 25 Difficult and 30 Impossible. Did I do something wrong? I don't know when I began to lose control....or if I ever had it....but this Rogue just became my new God.

Now, my confusion begins when it comes to the research I tried to do on this matter. I found another question thread here that gave me a bit more insight into traps and such. If I have a trap with, say a DC of 15 to locate, what does that passive 21 actually show him? That something is amiss in that area and he still has to roll to locate the actual trap mechanics? Then he also has to roll to try and disarm it if he figures out how to do such? Or does the passive perception let him see the entire trap and its workings? Secondly....I imagine only HE sees this trap, or the disturbed area of ground or whatever he may perceive to be wrong with the area, yes?

My follow up question is how does the Passive Perception affect Stealth/Hide abilities? Like if I wanted to set up an ambush of Dire Wolves with a Stealth of +4 . Some are visible, and I want to preemptively Stealth/Hide a few so that they're already hiding from the group when they come across them. What happens the moment good old observant Quist happens into the area and spots every single wolf, hidden or not?

I appreciate everyone's patience in dealing with my potential ignorance in this matter. I'm hoping my math is wrong but....it seems not to be. Right now Quist is sitting at a Passive Perception of 21 and a Passive Investigation of 22. How do I make that not game breaking; or throw them into EZ Mode :D

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    \$\begingroup\$ "and let's not forget the Rogues Expertise ability: double your proficiency bonus when making checks with two chosen proficient abilities + thieves tools." – Just in case there is any confusion here, the thieves tools aren't automatically one of those he's expert in; it can be chosen instead of one of the two proficiencies. \$\endgroup\$ – Paul Hutton Jul 20 '15 at 3:32
  • \$\begingroup\$ The question here can even be applied to my game in 3.5, I'm glad you asked this \$\endgroup\$ – Zakier Jul 21 '15 at 13:12
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RAW the Rogue does receive the benefits of Expertise to his passive.

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster. Here’s how to determine a character’s total for a passive check: 10 + all modifiers that normally apply to the check. - Player's Handbook P.174

The player has invested in making this character good at this, this is not a bad thing.

Rogues in general are supposed to be good at this type of thing, hence the proficiency bonus class feature and the player specifically spent a feat on making his passive perception better. The player could've spent this on another feat to specialize in another area (or gain a combat ability). This is a good thing, while it may make it difficult for the Rogue to be surprised by a trap or an ambush you should in no way try to outmaneuver this.

Don't worry, there are downsides to the Rogue for this.

To reliably spot traps and ambushes the rogue will need to be at the head of the marching order so that he has clear sight lines. Rogue's are not particularly hardy and the party as a whole is inviting more risk for this reward. Likewise while he is extremely excellent at spotting physical dangers, the Rogue will not be able to detect magic wards and other dangerous enchantments and may equally blunder into them.

Ways to handle his detection of a trap or an ambush

You are correct in assuming that only that PC has seen the trap/enemy. Unless he has some mode of telepathic communication he will need to speak out and announce the threat to everyone. The best way to handle this sort of thing is to pass notes or send text messages to the player(s) able to see/detect the issue and leave it to them to react and tell someone else. Intelligent enemies will see/hear the rogue warning the party and the combat should start immediately (no surprise round though).

The PC does not become omniscient of the trap upon detecting it.

The PHB itself is very, very vague on what information is received when a PC detects a trap. However, the DMG does have a nice little section about traps, their detection, and disarming them:

If the adventurers detect a trap before triggering it, they might be able to disarm it, either permanently or long enough to move past it. You might call for an Intelligence (Investigation) check for a character to deduce what needs to be done, followed by a Dexterity check using thieves' tools to perform the necessary sabotage...

...In most cases, a trap's description is clear enough that you can adjudicate whether a character's actions locate or foil the trap. As with many situations, you shouldn't allow die rolling to override clever play and good planning...

...Foiling traps can be a little more complicated. Consider a trapped treasure chest. If the chest is opened without first pulling on the two handles set in its sides, a mechanism inside fires a hail of poison needles toward anyone in front of it. After inspecting the chest and making a few checks, the characters are still unsure if it's trapped. Rather than simply open the chest, they prop a shield in front of it and push the chest open at a distance with an iron rod. In this case, the trap still triggers, but the hail of needles fires harmlessly into the shield. - Dungeon Master's Guide p. 121

Essentially though whether trap disarming is simply a dex check, a series of checks, and/or involves serious RP is up to you as a DM. I would encourage the open-ended approach the book suggests as it adds complexity and makes trap checking and disarming a more engaging process. Sitdown with the party OOC and discuss what the table as a whole thinks should happen for trap checks and move forward based on that consensus.

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    \$\begingroup\$ One way to apply this answer to the dire wolf ambush scenario in the question, when the party enters the area, have them roll initiative. Quist would not be surprised, but the rest likely would be. You can use the initiative order to see if Quist had time to warn everyone and prevent a surprise round. If he goes before hidden wolves, he can negate their surprise action. Of course, if he doesn't share info, run the surprise round as normal. \$\endgroup\$ – Evermeet Apr 24 '17 at 13:16
  • \$\begingroup\$ @Evermeet If you don't notice a threat, you're surprised. Being told about a threat is not the same as noticing it, so if the other PCs have not noticed the threat through their passive Perception, they would need to take a Search action to spot it. But they can't take a Search action on their first turn because they're surprised. However, I might be generous and let them use their reaction to make an active check after their first turn, but they would no longer be surprised by this stage so it's a moot point. \$\endgroup\$ – Clearly Toughpick 2 days ago
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As the DM, you determine when to roll stealth vs. perception. You also determine what it means when perception beats stealth.

Consider, for example, that even a tiny spider only has a finite stealth modifier. What do you do if a player character is looking in one direction, and a spider crawls up behind him? Do you have the spider roll a stealth check to see if it beats the player character's passive perception? If perception beats stealth, then what does that mean? That the PC suddenly knows exactly where the spider is? That the PC hears the spider? That the PC has a vague sense that something is behind him? That the PC turns around for no reason and conveniently sees the spider?

I would say there is no reason to roll stealth vs. perception until the spider is actually doing something that normally might cause it to be detected, like crawling up a nearby wall in the PC's peripheral vision. In that case, I would roll stealth vs. perception and then let the circumstances determine what information I give the character. If the lighting is good and the spider contrasts with the wall, then I might say "You see a spider crawling up the wall." But if the lighting is poor and/or the spider blends with the wall, then I might say "out of the right corner of your eye, you could swear you just saw something moving up the wall." And I would give the spider a bit of movement and then let the PC make an active perception check if he wants, asking him to specify exactly where on the wall he is leaning in to look carefully. Maybe the spider has already moved away from that point.

There are many things that can be detected, some more specific than others. You can see a creature's eyes through the sparse foliage of a bush. Or you can just see "something moving" behind/inside the bush. You can hear footsteps right behind you and feel someone breathing down your neck. Or you can hear metallic footsteps and the clanking of armor echoing down a twisting corridor and you have no way of knowing which direction the sound is coming from. Whenever you roll stealth vs. perception, it should be because you can concretely describe a phenomenon that might be perceived, and you need to determine if it is, in fact, perceived. If you can describe it, then you can determine the consequence of it being perceived.

To apply that reasoning to your wolf ambush: what are the wolves doing before they attack? (If they are just standing silently, then there is no reason why the PCs should be able to hear them, and therefore no reason to roll stealth vs. perception, unless a PC gets within a foot or two of a wolf where it becomes physically possible to hear the wolf breathing.) From where do they attack? Do they wait behind bushes until the PCs get within 5 feet and then jump out? (Maybe roll stealth vs. perception to determine if the PCs visually notice the wolves behind the bushes.) Do they silently wait 30 feet away, out of the PCs' line of sight, until they hear the PCs, and then they jump out and run at the PCs? (Then there is no reason to roll stealth vs. perception until the wolves are actually approaching, and even then, only roll for "you see something out of the corner of your eye" if the wolves truly are approaching from the sides or roll for "you hear panting and padded footsteps" if they get within a few feet. And even then, all it does is prevent the wolves from getting a surprise round against the PCs whose perception beat their stealth.)

Or traps in a dungeon: what exactly does the PC notice? A pressure plate? A tripwire? Then just describe what the PC sees (e.g. "one of the tiles in front of you is out of alignment with the others") and leave it to the player to investigate.

High perception does not mean you always know everything about your environment. It simply means you notice sensory phenomena that others do not notice.

Also, a house rule I like to play with is that passive skills are nonexistent. Instead, the DM keeps a cheatsheet with the players' skill modifiers and advantage/disadvantage. When an event happens and the DM needs to determine if a PC notices it, he silently rolls perception for the player (adding modifiers that normally only apply to passive skills, like the Observant feat) and stealth for the other creature if applicable. If the creature's stealth (or the trap's DC) beats perception, then the DM says nothing.

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I've played 3.5e for years and I just switched to 5th. I've had a similar problem, so I used the take 20 for a skill from 3.5 then add my monsters worst hiding bonus from the group. The reason I do this is because if the monsters take time to prepare this ambush then they selected a place to attack and get ready and just prepare in general or maybe it's the monsters home area and it's found a great spot for ambushes. For example let's say I have a group of Gnolls and one Gnoll Pack Lord. Gnoll pack lord would be the taken 20 +2= 22, however a Gnoll only has +1 so the Gnoll would be a 21, therefor I would use the Gnoll's DC against their passive perception. Then if it's an advantaged area for the Gnoll you can -5 off , for example, the rogues 21, see PHB pg 177 in Hiding

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