Sitting in Lexus Southampton today whilst my motor was recieving its "intermediate health check" :P when a bloke came in and complained that his windscreen had steamed up.
He was very persistant and said that he had his ventilation on the correct setting. I reckon, and so did the service manager, that he had it on recirculating air with the air-con off. When asked if this was the case he immediately went all defensive and the following burbled from his mouth: "It was on fresh air setting, my Ford and my BMW don't do it when they're on that setting" I had a little chuckle to myself as he got more and more irrate.
Eventually the service manager agreed to have a mechanic look at it, although you could see in his mind he was thinking "I wish you'd read the bloody owners guide"
In my experience, and drawing upon my extensive engineering knowledge , it is quite normal for the screen to steam up if recirculating air is selected and the air-con is switched off. Warm moist air drawn from the inside of the car and blown up the inside of the screen + cold air on the outside = foggy windscreen. Hit the Air-con button and the air is dried, no problem.
"If you're Ford and BMW don't do it then you drive them and let someone who appreciates quality, and who can read the owners guide, drive the baby Lex" (quality if you don't mind having CRAP PAINT).
By the way, if you are reading this and thinking "that was me", no hard feelings, I just found it quite funny.