I had fixed up a bike for a friend's son. (cheap repainted huffy MTB).
I got a call he had headed out of town and left the bike locked in front of the public library.
I got the combo wrong or something and couldn't get it the first day when I went to pick it up, so I went home without it. I came back the next day to pick it up.
And... the bike was GONE
Walking around the block in dispair, I found it unlocked at the bus stop 1/2 block away.
I never found the lock, but I think I did get both the bike and helmet.

Somebody must have come in on the city bus.
Taken it for some errands, then took it back to the bus stop for their trip home.