What do you call a Q&A user who posts a question but never checks back?

This kind of user is called an ask-and-run.

It is even mentioned on Meta Stack Overflow: Dealing with “ask-and-run” questioners

Bonus: If we follow the same pattern, we can also come up with a specific term ask-and-idle for users who post a question but stay idle (but don't disappear/leave) without accepting an answer, commenting, replying to people, etc.

There is also the term Hit-and-run posting (thus hit-and-run poster) but it is a more general term and it is usually used for a one-off forum posting posted by a hit-and-run poster that ignites discussion. The motive of a hit-and-run posting is usually flamebaiting (posting a provocative or offensive message).

Hit-and-run posting refers to a tactic where a poster at an Internet forum enters, makes a post, only to disappear immediately after. The term comes from the hit-and-run crime on auto vehicles, in which the driver hits another car or person causing an accident and runs away. It is also known as making a "drive-by" posting, a play on the phrase drive-by shooting. The post often consists of a lengthy text making lots of claims that can be, but are not always, on topic.

http://en.wikipedia.org/wiki/Hit-and-run_posting

These users appear for a short time before disappearing forever, resembling the behaviors of virtual particles in quantum field theory, and therefore I would like to nickname them Q&A fluctuation: the temporary appearance of contribution out of empty space.

They are, of course, known to their former teachers, relatives, and friends as "Easily Distracted".

"I don't remember asking any question"?