OK away from my Audi and onto my daughters car, a Rover ZR.
The problem is loss of cooling fluid.
I have carried out a 1.5 bar pressure test on the cooling system and there
are no signs of water egress. The hardware used could also show if the
cylinder head gasket was leaking by a pulsing of the clock needle, again no
pulsing was evident.
The only noticeable thing is that a few fins, maybe over a 20mm x 20mm area
at the top, on the radiator tubes have disintegrated. There is no water
leaking at this point.
All hoses look sound and there is no water on the ground at any time.
Yet, 2 pints a week are required.