There are basically four possible outcomes to an encounter with a trap:
- It goes off without being detected first
- It is detected and disarmed or circumvented
- It is detected, but someone cocks up and accidentally sets it off anyway
- It is undetected, but does not trigger.
If the trap surprised the party, it was undetected. If the party surprised the trap, they avoided triggering it. Most traps will have modifiers to the party's surprise roll for their construction; thieves receive a bonus to their surprise rolls against traps. In the event that the trap is triggered, only characters in front of the first character to detect it may be threatened (ie, if you have a character with a +2 surprise modifier in the middle of the party and you stumble over a trap that he detects, the front half of the party is in jeopardy if the party fails its surprise roll to avoid triggering the trap). Resolve who gets got in a manner consistent with the nature of the trap and trigger.