How do you do passive checks?

We played DW for the first time with my friends. All in all it went good, you can read about it some more here.

One particular problem we faced was when one of the NPC-s tried to sneak up on the party while they were resting. How do you handle that?

Do you create a custom move, something like "When someone tries to sneak on you, while you are guarding something, make a Discern Realities move" ?
Or do you just avoid situations like that?

Or how?

Ask them what they're doing while they guard the thing.  If it involves a move, deal with the success/failure.  When they run out of things to say/do and give you the blank stares then you're ready to make a GM move.  Announce future badness, tell them they hear a noise but don't see anything.  When they investigate, have them discern realities.  Reveal the NPC if they succeed and have the NPC dash in (and get the thing/push the button/occupy the coffin/whatever) before they can react if they fail (then let them react).

Yup, just like that.

Try not to think of NPCs having 'character moves' as the players do. They are the narrative tools through which you make your GM moves. Think about why you want the NPC to sneak up on the players... Were you thinking off screen? Are you trying to answer a stake question through the NPC? Is it a means to advance a grim portent? Is he just a danger of the location or front?

Whatever it is use that principle to drive your move (but don't state the move as you make it). Play a set up move first, say the hard move you want to do is take away the player's resources (the NPC is a thief). Sure you could just play that straight up as the players are making camp, but its a mite harsh and not very interesting as developing fiction. So you might reveal an unwelcome truth as Mease suggests, with the follow through on a player miss being to take away their resources or put them in a spot.

Just make it about choices and moves snowballing from the choices. 7-9 results are your friend!

Thank you both for the insight. Actually, now that it's written down it seems so evident and obvious. But then and there i just didn't know what to do. Guess it comes with practice.

To put it into some context, the sneaking npc was the goblin shaman, trying to get a drop of blood from the wizard character. I added a front to the scenario, that the goblins want to magically bind Florimel in order to use her to help them liberate themselves from slavery. I decided that the ritual needed a drop of blood from the wizard, being her lover once... So, in a way, i think this was announcing future badness, though we didn't explore this further after they killed Florimel very quickly.

Maybe this stealing a drop of blood isn't really necesarry at all, i could've just made a grim portent and have the goblins bind Florimel at some point.

Hey Roland,
That's awesome! I like your ideas, and thanks for giving us the narrative grit behind your mechanical choices, that's what drives the game right?

So that's a cool front stakes question... Will the goblins magically (or otherwise) bind Florimel to them so they can escape slavery?

You can 'frontload' this stake in many ways, remembering that YOU can't decide the outcome (its explicitly NOT a pre-determined GM plot). Putting the player in a spot via the thieving shaman is great, and if they succeded, you could 'think offscreen' and relay a little cut scene where the gobbo's are performing the ritual, or you may 'state the consequences and ask', by telling the players they know about the goblins ritual (through many evolving fictional means) by a pleading slave or a self-serving turncoat or finding clues that detail the goblins plans, or by simply by stumbling onto the ritual itself. What do they do now?

The drop of blood is a cool idea, but it could be anything (grim) yeah? In terms of investment and re-incorporationits even cooler when the players come up with the answer. So when they capture the shaman and interrogate the cowering prisoner, successful with a strong hit on their Parley (using a death threat as leverage)asking 'what the devil are you trying to steal from the wizard?' Respond by throwing the question back on the players, "I don't know, what's important enough from her to be used in a ritual to bind her to the goblins for their own ends?"

That way the players get some narrative control of intrinsically just a MacGuffin, but they are also handing you future golden opportunities on a plate and all sorts of plot hooks too. Note that you have delayed answering your stake question (thereby increasing the dramatic tension), and could follow up with any number of your GM moves. All good stuff.

Let us know how it all goes next time you play huh?