How can a question be closed by a single (non-moderator) user?
I've noticed a few people closing their own question as "off-topic", like here, here and here. I didn't even know you could close your question by yourself, and it seems strange that it's happening at all.
Did I miss some new announcement?
Edit: Actually I see that these are very old questions, so now I'm even more confused. Also there are many more examples, just do a search on SO for closed:1.