He thought he was alone in the house (save for the possibility of the handyman - who has a key to the house).
You're right not to spin about it -- you ought not spin about anything, really. But I have to admit I do find this odd. When I'm alone in the house I don't lock the door. Hell, I don't even shut the door to the water closet. The only reason to lock a door is to ensure privacy. Maybe there was a naughty monkey that needed spanking or something....
