ive noticed this too, ive put it down to the cold, when booting it sometimes it completely clouds up my rear window lol
Reason for me doing a search now on "exhaust"+"smoke"
I'd been driving about 15mins, 15 miles on Sunday, got to a roundabout floored it and there was a mass of exhaust cloud behind me - and it was the back screen that completely fogged up, it was def steam/condensation of some kind as it cleared off the screen very quickly.
Didnt do it on the way back though.
I was nervously planning on checking the oil filler cap tomorrow!
Sounds like a normal thing by the amount of other posts in other threads.