I have seen a ghost, the same one, twice.
I was living in a squat in North London and in my bedroom, I had tied a long piece of string to the light switch, so I could turn the light on and off without having to get out of bed.
I was asleep and all of a sudden I woke up and I opened my eyes and saw a man, wearing a motorcycle helmet and a plastic anorak, standing in front of my closet and going through my clothes which were on hangers. I could hear the rustling of the plastic anorak as he kept going through the clothes. I reached for the string to turn the light on and he disappeared in the blink of an eye. There is no way he could have been a human thief, he wouldn't have had enough time to leave the room in the blink of an eye, to walk or run to the door and to open it and then to close it behind him even, as the door was closed.
Then, a few months later, I moved to a bigger room in the squat, which was right next to my old room.
One night I opened my eyes and the same man was standing in front of the fireplace, facing it and funny enough he was smoking a cigarette, he was holding it and I could see the smoke coming out of it. One blink of an eye later he was gone.
I assumed that he was somebody who used to live there and who died in a motorcycle accident...