I used to get this a little bit now and again ...
Now it just seems to happen after I get car / windows cleaned inside - so probably some spray of some sort.
Found using aircon used to clear it quite quick but then would be steaming up again once stopped for a while.
So, now generally put aircon off, use outside air (ie. non recirculating) directed to screen / floor and tilt the sunroof.
Seems to work better for me ...
TG