knock knock - who was there?

Something pretty scary happened last night. At least I thought so.

After dinner me and Dylan went over to our neighbor's house. They were having a little BBQ out back and some beers. But since it was kind of chilly out and I wasn't drinking, I had very little patience with the one neighbor's love of talking only about himself and calling everybody either bitch or fool. So at like 10 I went home. I left the door unlocked since I didn't know if D had keys or not and it's not like we live in the ghetto.
Then I woke up at like 1 am from the sound of the door opening, I know I wasn't imagining it because the cat ran out to see who it was too. But assuming it was D I just laid in bed half asleep expecting him to come in and start brushing his teeth and all that normal stuff. But instead I just heard some footsteps in the living room and then the door was shut again. I got out there, the cat sitting in from of the door looking like "who was that?" and man do I wish she could talk!

But he/she/it, who ever it was, was gone. I called D who didn't pick up, texted him too, but no answer. So I locked the door and went back to bed, obviously unable to fall asleep. Half an hour later D comes home and has to knock on the windows for me to let him in. He said it wasn't him who had been there earlier (I had told myself that maybe they'd decided to go to the bars and he had just come back for his wallet or something...) so it remains a mystery. But from now on I am locking the door.