Back in the day my mom and I were always the last to go to bed. One night I had been lying awake for a few minutes, when I heard my brother (who was about 11 at the time) suddenly start screaming hysterically outside my room. I quickly jumped up and ran to him, and found him sobbing and shaking in the passage. He told me someone was in the house, so without really thinking I started running around turning lights on trying to find whoever he'd seen, but, as far as I could tell, there was no one.
I went back to him, and he explained that he'd been woken up when we started closing doors, locking up etc, and went to the bathroom. When he was walking back he saw a woman (who he had thought was my mom) walking towards my parents room. He called out to her to say good night, but when she turned around he realised that it wasn't actually her...
According to him the woman gave him a weird 'who the hell are you?' sort of look, before continuing down the passage towards my parents room and just kinda disappearing into the dark. Which is when he (understandably) started freaking out.
A few months later we found out his ADD medication was making him paranoid (a rare side affect apparently), so I guess that's the most plausible explanation, but we were all pretty shaken up about it, and it took him ages to get over it, poor dude.