Curiosity. I once dropped my keys on the way to my leasing office. I searched the entire complex and office for my keys. Then I saw a guy at the mailboxes trying to open each one, one by one.* I asked if he needed help and he just said he found some keys on the ground and wanted to find out who they belonged to. They were mine. And my mailbox was in the other side of the complex so all bets were off for him anyway.

It costs next to nothing to try out a key in multiple places in the same proximity. Once you start going door to door using a random key you found, that's suspicious.

*it occurs to me now that I write this that this behavior is suspicious as well and probably illegal. He should have turned it into the leasing office.