Triggering "perception" things

I'm not sure what to do when a character would normally "perceive" things. Traps, for example. I know that the Thief could be looking for those things, but what if she's not? What if there is no thief, and the Wizard stumbles onto a trap? Do I give them any chance to find it?

What if it's not a trap, but a secret door? Someone hiding in the shadows? Etc.

So you have the trap as part of your prep (or a secret door or a monster with the instinct 'hide in the shadows') yeah?  Its written there on the map and in your notes and  everything. Just remember that it isn't part of the fiction until its authored in...

So follow your principles, build the tension by giving signs of impending doom, make a hard monster / dungeon / GM move on a missed roll  by the players and spring that trap / attack from the shadows on the players if they aren't looking out for it.

It isn't so much 'I make a perception check to look out for traps', but rather; 'I scout about carefully around the cavern, lighting every nook and cranny with my torch, being careful not to rush, using all my senses to make sure that the area isn't hiding any nastiness.'
'Oh, OK, cool. Sounds like you are discerning realities to me, roll +Wis'.
'Sweet.... 7-9. Hmmm. What should I be on the lookout for?'
'Well as you examine the flagstone floor near the far door, there are suspicious scraping marks around the edges, as if the flagstone has recently been moved...'

How are they used in movies?  They're usually soft moves.  You step on the mine and hear the click, what do you do?  6-, that's too bad, you see Chevar, the hireling in front of you, go down - a dart protruding from his arm (now you know there are traps here).  What do you do? You see the grenade on the door but the bad guys are closing fast - what do you do?

Characters don't normally "perceive" things in Dungeon World. They do stuff that reveals things, or the GM reveals things with a move.

The players may find traps through clever plans, trap sense, or discerning realities. If a character describes an action that doesn't trigger a move, but the action would still discover a trap, don't hide it from them. Traps aren't allowed to break the rules. "Traps," Page 173.

I was wondering about this one too. I think there are (maybe?) two ways of approaching traps. I think one way is where the trap "didn't exist" until the player rolls a Partial Success or a Failure and prompts the MC Move. So, if the player is searching a room and gets a failure, the DM can retroactively "add" the trap to the dungeon and spring it then.

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?

Let them roll what?

I don't know. I mostly play World of Dungeons. Translate what I said into Dungeon World Moves.

Discern Realities would be the obvious answer.

To do it, do it.
'To make a move', the player has to fictionally author in 'OK. So what do you do?' responses that engage the move triggers... Otherwise you are just having the conversation of roleplay. Moves follow the fiction.

Appropriate narrative triggers in this example I can think of (whether or not the trap is in the prep or not)...

When you consult your accumulated knowledge about something,
(Spount Lore)
When you closely study a situation or person,
(Discern Realities)

Or, once the trap is discovered or sprung...
When you act despite an imminent threat (Defy Danger)

Noofy's going where I was going. It's not about letting them make a roll. They do it "We're cautiously moving down the corridor, keeping our eyes peeled for danger. I'm peering ino the shadows and studying the floor before I move." GM: "Make a discerning realities roll."

They get to make that roll regardless of whether there's trap, because they're discerning.

1) Be a fan of the characters.
2) Always say what honesty demands.

There is nothing cool or interesting about a secret door that the characters don't find.

Or, as Vincent Baker put it back in Dogs in the Vineyard, having secrets isn't cool--revealing secrets is cool.

If there's something there, I want the players to find it. Of COURSE they find it. They're the stars! The question is HOW do they find it and WHEN.

In some cases, it's because of Discern Realities. In some cases, it's just straight cause of the fiction. And sometimes it's from some other roll.

But with a straight-up there's a secret door in this room? Of course they find it. Give it to the thief, or an elf, or a dwarf, or whoever's got a high Wisdom, or whoever's been out of the limelight.

The question is, what do they do with it?

Or have something come through it ;)

"Let them roll" I guess is a bad choice of words - I didn't mean to say "give them permission to roll". I meant, if they're searching, then they're "doing it" and should roll, so "have" them roll, not "let" them.

So, from what I understand, if they're actively searching for the trap, they'll get to roll Discern Realities. If they spring the trap, then they Defy Danger. Is there any circumstance where a Defy Danger roll is not granted due to the fiction? If so, what's an example that would give the GM a golden opportunity to hit them with a trap hard move?

I could maybe see Spout Lore being used if the player says something like "Have I heard of any legends of this place having lots of traps?" Otherwise, how could that Move apply to finding traps?

If the players describe an action that would discover a trap, they don't need to roll. That makes sense. If the player says "I poke the statue's shield with my ten foot pole" and that was the trap trigger, then boom, done. They find the trap and probably lose the pole. No roll needed.

I kind of agree with the statement that a secret undiscovered may as well not have existed, but I'm not so sure I totally agree the entire comment. Are you literally suggesting that if the PCs enter a room and ask if there's a secret door there, they should just find it automatically? That doesn't follow the fiction. You'd have to say "How do you look for a possible secret door?" and then they'd roll to discern it or they'd describe their character looking in the dead-on correct place and discover it without a roll.

Is there any circumstance where a Defy Danger roll is not granted due to the fiction?

I'm not fully sure, but in AW I had times when I just made move "inflict harm" without any option to defy danger, when character obviously did not see the danger.
Remember, to DEFY something, you have to KNOW about it.
It is legal imo, though shouldn't be done lightly or smth, ? take care. First trap in the dungeon when heroes didn't even remember to look for traps? let them have some damage, not A LOT, but enough to get them into the mood. After one such thing they'll play nice little stalkers in no time.

Sure, let's say you've established the presence of the trap (revealing an unwanted truth, show signs of an approaching threat). The Wizard decides to search the room and rolls a miss on his discern realities. The GM can totally decide to spring the trap and deal damage at that point. Make as hard a move as you like right?

Speaking of traps—keep your eyes open for them, too. The thief is your best friend there. They can stop you before you wander into a pit trap or fill the room with acid. Without one you’re not in dire trouble, but you’re likely to need to take your time and be extra careful. You can investigate an area by discerning realities, but you’ll be taking more risks than a skilled thief would.
When you’re unlucky enough to trigger a trap you might have a chance to get out of the way, throw up a quick protective spell, or save a friend—most likely by defying danger. Of course not every trap is so crude as to give you time to get out of the way. A well-built trap will have a blade in your side before you even know it’s sprung.

p.40
So yeah, sometimes the GM makes a hard move and they decide to reveal an (hitherto unthought of) unwelcome truth and plant a blade in your guts from a trap... Now that is HARD. But I love it.