Had the same experience in my S3 and it was simply and definitely an air-con issue. I had a similar thing on my 1989 Renault GTA V6 many years ago (still have the car but the air-con is currently out of service.)
In my case I had driven the car about 150 miles (to the factory) using air-con all the way. Parked up and then re-started about 15 minutes later. The car immediately filled up with steam and with windows open and fans on max this cleared in about 5 minutes. Not a leak specifically and not anything other than an air-con quirk as far as I can see. What the longer term prognosis is I have no idea but I doubt it helps with the long-term health of the air-con.
