I'm assuming that, in @zakael19's example of partners finding one another, it's mutually accepted at the table that it's not to be taken for granted that this will happen. It may, or may not, and the mechanical framework is used to find out which.How do you know if it is a challenge? Do you frame the scene with the challenge in mind, like 'this is a skill challenge scene about x'. Or do you talk about the character goal on a meta level, like the player may say 'my characters wants to achieve x' and so then you frame a scene?
In the example that I posted, from my Middle Earth game, I as GM framed the scene with the Uncertainty scene distinction. I won't have done this arbitrarily, so although I don't have any record of the details I would guess that the players were discussing among themselves what to do next. The circumstances of play revealed reaching a decision to be a challenge for the characters.