We apparently have an extra cat.
This one doesn't eat anything, nor try and trip you up while you try and negotiate the house in the dark.
Middle child (she of sanity and reason) decided to sleep in my bed last night. She had been in there for some time when she come flying out and slammed down in the seat next to me at the computer. She said that one of the cats kept walking over her, kneading her and generally being a pest, so she turned on the lamp to see which one it was and there wasn't anything there. She could feel it sitting on her legs, but couldn't see it.
She got the shivers so shot down under the blankets and tried to ignore it. It walked up to the pillow and started kneading that right beside her face. She could hear it purring, feel the pillow shifting but again couldn't see it.
The funny thing was, she wasn't willing to sleep in her bed at all then. Insisted on sleeping with me. I don't know what she's going to do when Mr DT gets back. He won't share space lol.
The fact that the cat is purring means he/she is a happy wee cat. Have you had a cat of yours die in that house? Could this be her guardian angel?