Firstly a bit of background on my house. I've lived here for the vast majority of my life except for a 3 year gap whilst at university. My parents bought the house when I was 6 months old from a group of elderly siblings after one of them died (the remaining 2 moved into a bungalow across the road). It's a rather old house, built sometime in the 30's during WW2 and still has an air raid shelter in the garden.
The encounter took place in the bathroom; however, this room was originally a bedroom and wasn't converted into a bathroom for about 8 or 9 years after we moved in. It was, however, in a great state of disrepair and we never used it as it was unsafe until we converted it.
I'm not sure of the date as this took place at least 10 years ago now, but it was still several years after we turned the room into a bathroom.
Anyway, it was rather late at night and I was stood by the sink (which faces a large window) brushing my teeth before bed. Out of the corner of my eye I thought I saw a greyish mist floating by the toilet to my left, but when I turned to look properly there was nothing there. I didn't think much of it but then moments later I got the sensation that someone walked behind me (from my left by the toilet) and outside the door to my right. And as this happened I got the very distinct smell of cigarettes; you know, the typical smell of a heavy smoker, which vanished after a few seconds. Now the window was shut (I checked) and my family are all non-smokers so no one had smoked in that house for well over a decade so there was no logical explanation I could think of.
The next day I mentioned what happened to my mum and she told me that the people who lived here before (the siblings) were all heavy smokers. I did not know this before the incident. I just knew that there were three of them and one of them died and that the other two still lived over the road.
I've always felt that this house was quite spooky and had heard the odd strange noise and such, but had never experienced smelling anything or seeing mists. And I've never experienced anything that big since.