I think my door was open when I came home and no one was inside. I thought I had locked it.
That's scary! Was the door completely opened or closed but not locked?
Someone had to re-lock the door. Nobody else has a key.
I'm glad that you are safe. You should always make sure you lock your door when you leave home. If not, you could be robbed.
Maybe there is a ghost in the house.
Unlikely, why would a ghost need to unlock a door? Just be more careful about locking the door.