What is the right way to GM magical trap detection?

Question

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?

Answer 1

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.

Answer 2

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".

Answer 3

For general perceptiveness type things, I use the character's Base Perception skill +10. But not for trap detection, secret doors or anything else secret. At the beginning of each game session, I have each player give me ten d20 rolls. I transcribe them to a master sheet so I have all in front of me. When I need a secret roll of any kind, I roll a d10 and pick their d20 roll and add their base perception score. I can also use these rolls for things like Fort Save vs poison, Save vs illusions, etc. Once I use a number, I cross it out and if I run out, I just get them to give me another ten d20 rolls.

I've been using this method for over three decades and it works like a charm.