Triggering "perception" things

If someone is just walking down a corridor in a dangerous dungeon, whistling "The Wizards staff has a knob at the end" that is what i call a GOLDEN OPPORTUNITY!

Okay cool, I think I got it.

Otherwise, if the trap was planned, then the players could find it in a number of different ways. Maybe it's a javelin throwing trap in the middle of the hallway triggered by a pressure plate on the floor.

If the PC's get to that hallway and say they want to proceed cautiously or investigate it carefully or whatever, then let them roll. On a Success, they find the trap before they spring it. On a Partial Success, they spring the trap but can react before it does any major damage. On a Failure, they spring the trap and eat the consequences.

If the PC's get to that hallway and just proceed without caution, I'd use the Partial Success option - they spring it but they can react before it hits them. If the PC's are getting chased or something through the dungeon and run wildly down that hallway, I'd just use the Failure option and hit them with it immediately.

Anyone disagree with those two approaches?

Reading back, this doesn't seem right. In the case where the PCs proceed without caution, I think a Hard Move is necessary. If you default to Setup Move, it makes searching more dangerous then proceeding without caution.

If "let them roll" refers to a Discerning Realities Roll, the 7-9 result means you get to ask fewer questions. It doesn't trigger a trap (and you get plus 1 forward I think). On a miss, triggering the trap is perfectly appropriate with the GM having the option of choosing their move, including putting them on the spot and asking what they do, or dealing damage depending on the fiction.

If they just walk down the hallway the GM can trigger the trap and has their pick of moves as above.

That's one of the reasons I asked what they're rolling. If they're Defying Danger it implies something very different about the fiction and results in a very different turn of events.