In D&D there's the notion of spotting secret doors, traps, etc. passively. How does one handle that in dungeon world? Discern Reality appears to be an active roll on the players part. How do players just happen to notice the pit trap because the thief catches a glimpse of a wire hanging off the trap out of the corner of his eye?
As you mentioned in the comment you probably need to shift your mindset a bit. I would think about what you are trying to achieve in the narrative with your trap. And then do one of two things:
If you are trying to use the trap as a way to convey that the location is dangerous. (This could make a good "Point to a looming threat" dungeon move) Then I would just tell the thief
As you are walking you notice a bit of wire hanging from the ceiling with no obvious purpose. What do you do?
The thief can then choose what to do. You don't need to roll for a perception check, and this is totally OK. Regardless of the outcome the party is now aware that the location is trapped, and you will have achieved your goal.
If you are trying to make the place actually dangerous (perhaps you've already shown it is dangerous some other way) I would let the thief look for traps without dropping any hints. If the thief looks for the trap (triggering Trap Expert) you can follow the move to its logical conclusion, otherwise you can spring the trap on them. Either way you have succeeded in making the place dangerous.
As a general principle for Dungeon World, never roll dice unless you've triggered a move that tells you to. If you find yourself in a scenario where you want to roll dice but no move has been triggered, take a moment to figure out what you are trying to achieve and how you can make it happen by following the GM's rules and making a fitting move.
There is no "passive" in Dungeon World.
The way a thief sees a trap is to take a second to survey the area, per the Trap Expert move. If they don't, there's no "well, a thief of MY level would certainly..."
Now, you're welcome to telegraph the possible presence of a trap so that the player uses the move. "There's little nozzles placed waist high all the way down the corridor..."
However what you don't do is "add little oddities to try to trick them into looking for the traps." That's backwards to how Dungeon World wants you to do things. In fact, the trap might appear or go off in response to a poor roll when they do Trap Expert. I know it's a little more "quantum state" than you might be used to, but you are not "putting stuff in making them roll all the time" like D&D. You skip the boring crap, and when there's a place with a trap in it, you point out a clue or something, let them roll, and if they fail why they get trapped and if they succeed they don't (in any variety of ways - they disable it, the nozzles were decorative, doesn't matter, whatever forwards the fiction).
There are great answers here, but there are 2 problems I see with any kind of passive spot hidden roll.
One is that the trap might not exist until the players roll 6-.
Example: the fighter rolls 6- after triggering Hack & Slash move because he swing his hammer at the heads of a squad of kobolds. Now the GM can have the party thief, who is hiding in the shadows, step in a bear trap anchoring him in place. The bear trap did not exist in the fiction until the fighter failed his roll.
The other problem is that unless the GM describes the area as dangerous or a previous player roll <10 allowed the GM to threaten a trap, then the GM is likely to never get a chance to spring the trap. GMs can only make moves on 1) players look to GM to find out what happens, 2) golden opportunity, 3) players roll 6-. So if the GM has not provided any fictional context for the trap and the characters have not triggered any GM moves, then they waltz right past the trap and there is nothing the GM can do about it. Or more appropriately, the trap doesn’t even exist until those conditions are met.