Happens to me sometimes at night, when I'm in bed. I'm an only child so I've never been afraid to live alone or stay by myself. I even forget to lock the damn front door when I go to bed, sometimes.
But on occasion, I'll be in bed with all the lights off, getting ready to go to sleep, and I'll think I see someone walk by my bedroom door. Not directly, but sort of like out of the corner of my eye. I have 5 huge pecan trees in my front yard, 150 ft tall or more, so it's probably a shadow from them, coming down the hallway. Or maybe nothing at all, but it still creeps me the hell out when it happens and my heart always starts to beat just a little faster.