I'm asking here because if this becomes a trend I'd like to know what I should do in the future. I don't have proof, but I suspect the user wanted a direct solution but didn't want their prof to be able to find it. It's cynical to make such assumptions, but in this particular topic there seem to be a lot more students who just want to pass a class than industry practitioners, and if one student figures out how to game a system others will follow.
I'm specifically referring to this deleted question which had to do with calculating the result of a decision tree in Prolog.
I had asked the user if the question was homework. The user replied saying was not a homework problem, they were self-studying, and why would I care anyway. I replied to say it was so I would know whether they'd like an indirect answer or a direct answer when using a direct one might constitute plagiarism in a class setting. I proceeded to supply the direct answer, because they said they were self-studying. I came back an hour later and the question is gone and so is my answer.
If this was the case, I'd be tempted to repost the question myself along with my solution as community wiki. But it wasn't a great question (it was just shy of too-specific) and since I'm not certain that's what happened I'm not sure it's an appropriate response. What to do?