I am a first time GM in a PBM game. One of the characters (the Thief) is following an NPC (another thief) and would like to not be noticed. They are in a port, among warehouses and depots.
I feel that this is triggering a move. The most fitting that I can find is the omnipresent Defy Danger (DEX variant). I feel that the Defy Danger move is being triggered because the NPC being followed is an expert thief (this has already been discovered in-game).
If the NPC were a normal guy, the Thief would be able to follow him without being discovered (since the Thief is an expert in such things, and a normal guy is not alert to being followed without a previous reason for being suspicious). So if my player were following a normal guy, I wouldn't reckon any triggered move.
But we have another expert in the art of spying and being inconspicuous. And so following him is not an automatic feat: there is a danger of being noticed.
Since this is PBM I have the luxury to think about it and not improvise on the spot. Is there some other idea that I am overlooking?