Coolant Level in Reservoir Audi Allroad 2002 AKE 2.5TDI

Hi all. I have been meaning to ask this question for a while.
About 3 months before I did my cambelt, water pump, tstat, idlers, tensioners etc change in March 2012, I got the warning on the dash that coolant was low - sure enough it was and I topped it up. This happened once again. Thinking it was probably a tstat or water pump leak, I was not too concerned as I was about to change them.
This job went well and car has now done 13k since.
One thing I have noticed - the coolant level in the reservoir sits bang on the minimum. If I top it up to the max, after a few days it is bang on minimum again, I can see from the "pink" coating on the exhauster pump underneath that the "excess" coolant is getting dumped out of the (new Audipart) reservoir cap. The car has run this way on minimum now for over a year - no coolant loss whatsoever.
I just wonder if any of you chaps experience the same? Trouble is, I have only started servicing the car from 140k (mileage now 160k), and never thought of checking coolant level for the first 140k, so I don't know what is normal for my car.

Either the cap is faulty ( O ring ) or you have a crack in the tank so when the water expands and rises in the tank it leaks out. Be carefull but next time the engine is up to temp check to see if the tank is pressurised. Regards Peter

Or simply check if the thick hose going to the radiator is pressurized, it will be bone hard if pressurized, otherwise soft