I read this article
about this cat that leaves the house every night and then waits at the same place every morning to be picked up. The cat called Sgt Podge disappears every night from his home and is always found the next morning in the same place around 2.4km away. He does this so often that his owners have a regular daily routine where they take their son to school and then go to pick up the cat. I think I would start locking him at night.