Do you live with anyone? Have any friends that know where you live? Most likely it's just someone like that trying to make you laugh/freak you out, I'd think. It's not hard to imagine someone with a ladder and/or a long stick, or someone from inside playing a goofy prank.
Or it's possible that kids were being dumb and throwing bagels at the building. Or even someone above dropping a bagel on accident as they looked out the window. In either of those cases though, you'd have to get extremely lucky...
Another option: the killer is still inside your house, and the bagel was a distraction. Now that you took time to post it on GAF, he/she will strike.
Also possible: you're facing mental health issues, and put it there without remembering.