My parents built and moved into a new house a couple of years ago, and this house has locks on the doors, but of course, it's still never locked.
Except, when I arrived this time, there was a key in the lock of the front door. On the outside. Just hanging there, somewhat mysteriously.
It turns out that my parents' dog is able to open the door and let himself out, so sometimes they lock the door to keep the dog in.