Just to add a bit more confusion into the mix, I've got the steamed up glass too, however my A4 B6 Avant also has condensation on the metal inner skin of the roof above the head lining. And it's not as a result of sunroof drain issues, I don't have one. (Sunroof that is!)
My air con is suspect, but I can normally 'free' things off (sticking vents) within a few miles, I'll be getting around to changing actuation motors when things warm up a bit, no garage so I don't fancy freezing! But I suspect my headline condensation started as a result of the air con issue.
My point is, that if there is already a hidden source of moisture behind a panel or under a carpet, then this will be suficient to steam up your windows when you leave the car, particularly if there's a bit of sun and then a cool afternoon/evening to get the vapour moving and then condensing.
If you can find the source that's half the battle. Then you just have to sort out the cure! ...that's where I am ...I REALLY don't want to have to take out my head lining, but what's my alternative?