So often there are questions that are answered in the comments. Standard MSE policy is that we post the comment as an answer in the form of a community wiki so that the question can be accepted by the OP.
However sometimes the OP basically does a "hit-and-run" and never reappears again leaving the question permanently "unanswered" (ie not accepted). Seeing that this is sometimes a problem, would it be useful to consider a "no longer relevant" flag to close such questions? Maybe we could impose some guidelines as well on when such a flag should be used.
EDIT: Actually I looking at this question I see Martin Sleziak's point (refer to comments). Can this be an example that falls under no longer relevant?
EDIT: Some questions don't have enough information in them to be answered. However flagging it as not a question doesn't seem entirely appropriate either. One such example is this question. A new "no longer relevant" flag would be appropriate for such circumstances, I think. I tried flagging this question as not a real question but my flag was disputed. This would remove a grey area in flagging.