My contribution to the ghost stories and then a few comments.
My wife and I have lived in our current apartment during 2 separate periods since 2007, 9 years in total. Around the first time, maybe 10 years ago, I was on the bathroom in the most distant bedroom of the flat (most distant from the entrance, the closest to the street) brushing my teeth during a particularly hot summer afternoon. I finished brushing my teeth and decided to freshen up by washing my face.
While removing my prescription glasses and just before bending over the sink, I heard my wife (we were alone in the house) entering the bedroom, the way you hear someone with bare feet walking in wooden floor, and "felt/noticed/heard" movement behind me. I continued washing my face (with my eyes closed, of course) and kept blabbing with my wife about a conversation we had had a few minutes before. Right then I clearly felt my wife walking up to me and lovingly hugging me from behind, I felt a cheerful sensation, rinsed my eyes and looked upon the mirror to establish eye contact with my wife to ask her if there was a special reason for the hug... and there was no one there.
I froze, the hugging sensation had vanished less than half a second before. I called out loud: Honey?!, Love, where are you? I walked as fast as I could and found my wife on the kitchen to ask her if she had ran out of the bedroom for some reason. As she laid eyes on me she frowned with worry (later she would tell me I was pale and looked scared, as pale as a brown man can be, paler than she had ever seen me), and told me she had been cooking for over 20 minutes and had not left the kitchen. She solemnly swears it wasn't her, because for some time I thought maybe it was a prank. But there is no way she could have left that fast without making any barullo, without running heavily.
We have a Pomeranian now, it is in love with my wife and follows her (like a lap dog) wherever she goes, except to that bedroom. To this day the dog still refuses to enter that bedroom. Whenever we are in there, the dog sits in the door's threshold eyeing us nervously.
This are called hypnagogic hallucinations, are pretty common and do not indicate any pathology. We observe this frequently in the sleep lab.
You beat me to it, it's a parasomnia called sleep paralysis and it's nothing to worry about. In México, the floklore calls it "se le subió el muerto" (the dead man climbed upon him/her) and usually causes a lot of distress, but nothing else. It is a transitory dysfunction in the neural nucleus (in the brainstem) that switches off the muscle tone in the body during the rapid eye movement phase of sleep (it should switch the muscle tone back on upon awakening or upon exiting the REM phase).