I tried the same thing of switching the heater on in the afternoon and nothing happened. I don't know if it was just a coincidence.
My audi A4 1.8T (2005) overheats in the mornings when I switch on the heater? on 2006 Audi A4
Must be the gauge circuit or another problem, heater is not actually the trouble. Have coolant level checked.
poss water pump impellar rotting away,very common