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I'm trying to understand the rules of perceiving magical traps, but I remain confused on some finer points.

Let's say the PCs will encounter a Symbol of Weakness trap. The trap is on a door, and is triggered if someone passes through the door. If triggered, the trap targets all creatures within 60'.

  1. When a party member approaches the door, does she get an automatic Perception check to detect the trap? Or does she need to actively search for the trap?
  2. Does it make sense for a non-magic user to perceive a magical trap, even when their Perception check succeeds?
  3. If any party member detects the trap, what must I reveal? Do I need to reveal it's a Symbol of Weakness trap? Is it sufficient to reveal it's a magical trap, so a magic user can attempt a Detect Magic to learn more?
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2 Answers

  1. By the RAW, no - the players must ask to make a Perception check to locate it. It's difficult to determine this in one place in the rules, but without this rule, the Rogue talent Trap Spotter doesn't make any sense.

  2. Yes - anybody with a high enough Perception check can find non-magical traps - this is in the Traps section of the Environment chapter. A Detect Magic might find it automatically, depending on the surroundings, and how the symbol is placed (for instance, a clever caster might place other enchantments on the door, to disguise the fact that there's a magical trap there, or perhaps the area was all created magically or from magical materials).

  3. If a Perception check reveals the trap, then you generally reveal that a trap exists, and possibly that it is magical or mechanical. For a magical trap, a Dispel Magic or similar could then be applied, but in both cases, a Disable Device attempt could be made to disable it. Note that failing this check by more than 5 sets off the trap, so the remover must generally do this without knowledge of the type of effect.

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Are you sure about Trapfinding being required to perceive magical trips? The two mentions of Trapfinding in the Environment section discuss needing Trapfinding to disable magical traps, not perceive them. –  Craig Jul 22 '13 at 5:20
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This source seems to disagree with you. –  called2voyage Jul 22 '13 at 13:35
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@YogoZuno: You've proven my point. A Disable Device check and a perception check are two different things. You need Trapfinding to disable a trap. You apparently do not need it to perceive a trap. –  Craig Jul 22 '13 at 16:04
    
Yup, got it wrong again...<sigh>my brain just can't keep that one in. –  YogoZuno Jul 22 '13 at 21:01
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1.- I usually make a hidden perception roll, unless the palyers state otherwise. Which is the size of the magical mark, or the trap in question? Give them a penalty to the roll if it's hard to spot. Where is it located? Secret rolls are my way to go, but if you don't like it, try to speak to your players and tell them you usually lay traps arround, and whenever they feel something they should be using their skills. D&D 4E threats perception and insight as passive skills, again, a secret GM roll would solve all of this without creating a GM vs player scenario.

2.- Depends on the trap. Is the Sign plain visible? Anyone without magic knowledge would recognize a sign, but that doesn't means they KNOW it's a trap. Let's imagine a Fighter with Knowledge (Arcana) skill, he's not a wizard, but he may KNOW the sign is a trap with a succesful roll. A rogue without mystical knowledge might spot it, but think it's a thieve signal and just pass by it, triggering the trap. It's a trap, it can be spotted, but it takes the players to analyze it and understand it, just as an automatic arrow shooter hidden in the nose of a Moai Statue, or a doll that will shoot needles one you pass by her side. If the sign is being hidden by some spell, then you would need more work and caution to spot it, and that would need the player to actively use a skill, not solved with secret rolls.

3.- Just say it's a symbol. If they decide not to check it, they'll trigger the trap. If they decide to be cautious, they might eb able to learn it's a trap. It's a elarning anyways, if they trigger it, next time belive me they'll think twice before ignoring a signal, and if you made a secret roll and warned them, they most likely won't blame you or call it "unfair".

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So, to summarize, you're suggesting: 1. Use a passive perception check rolled by the GM. 2. If any PC succeeds, they can see the symbol on the door but may not know it's a trap. 3. If the PC has a magic skill, offer them a check to determine that's it a trap symbol. –  Craig Jul 21 '13 at 20:48
    
1.- Yes and 2.- Yes, but about 3, I'd rather let them decide what to do, you already gave them a handy secret roll. If they want to detect magic on it or use their memory to remember they saw it before on a mystical encyclopedia, it's up to them. Let them decide what to do once they found it, it's a trap anyways ;) –  Aldath Le'Carde Jul 21 '13 at 20:51
    
I see. But for 3, your notion is that a PC without magical knowledge or skills wouldn't qualify to detect a magical trap? –  Craig Jul 21 '13 at 20:52
    
Yes, everyone can detect a magical trap if they know how to do it. What I mean is that a fighter with some myscical knowledge can notice it, then use his brains (his Knowledge skill) to know it's a magical trap, with a little luck he might even say which kind of magic it triggers. A wizard can wither use a detect magic spell to know what the mark is, or just use a Knowledge skill to recognize it. As long as the trap isn't Concealed by magic, any PC can detect or even know its "nature". –  Aldath Le'Carde Jul 21 '13 at 20:59
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@Aldath: As I pointed out to YogoZuno, he's wrong about Trapfinding. Trapfinding is only needed to disable traps, not perceive them. –  Craig Jul 22 '13 at 16:05
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