There is a light over the stairs that leads up to his front door. The door is glass and has a translucent shade on it. If someone is outside the door at night he can see their shadow on the shade.
One night he happened to glance down the hall leading to the door and saw a creepy looking shadow. Someone was standing outside and close to the door. Thanking he had maybe failed to hear their knock, he went to the door and peeked to see who it was.
The instant he looked out, the shadow faded away and there was no one (or thing) there.
He didn’t sleep well that night.