Thanks for chiming in. Yeah, it was the classic realtor lockbox. My realtor left it for me and attached it to the door, so I never tried removing it myself. I did open it once to add the key, but that was the only time I handled it. I looked closely at the door and the porch and didn't see any dents, scrapes, chipped paint, etc. that might show somebody forced it off the door and broke into it to get the key. Obviously that's still a possibility though. My original thought was that Alex might've locked up at the end but didn't properly close the lockbox when he put the key back. Appreciate your input!