In my house there is a door in the kitchen that leads to the stairway to the upstairs apartment. The computer is almost directly across from this door, right outside of the kitchen. There is another door at the foot of the steps (it has its own hallway too) that leads outside. Whenever someone comes in that door, you can hear the opening and closing of the door. Whenever someone sets foot on the steps, you can hear the sound of their footsteps.
Sometimes, late at night, I'll sit here on the computer. I hear someone stomping up and down the stairs. But when I turn around, no one is there.
On night my cousin was lying in her living room, which is right at the top of those steps (with a door at the top of course). She was waiting for her husband to get home from work, who was working night shift at that time. She heard the outside door open, and heard him come up the steps. It seemed as if he was just standing outside the door waiting to be let in. This made no sense, seeing as he had keys, and the door wasn't locked anyway. She got up to check. When she opened the door, she was startled to find that no one was there.
The next day he told me about this. I replied that around the same time that night, I heard the same thing. I thought it was her husband returning from work too, except I didn't hear his car in the drive.
Another time, she was sitting in the living room with her one year old daughter. Her daughter had just started speaking and was repeating everything anyone said to her. My cousin had just started to doze off when she heard a woman's voice, and then her daughter saying it too. (They were home alone.) When she asked her daughter what she had said, she replied "What the lady said."