Does anyone live with you that could have been pulling a prank on you? Or if you left the keys around for your buddies or someone else to mess with that could be it too lol. If not maybe it is something weird, I've read on the Internet about random objects appearing in their home. One guy who lived alone had a spoon he never had before appear in his silverware drawer and he knew it was different from the others because the rest were a set that all looked the same.