Mk 5 Golf TDI Coolant loss

I have a 2006 mk 5 VW golf sport (105bhp) which has been showing 'check coolant' warning on the dash. With no obvious signs of a leak I had it pressure tested and was reassured that it wasn't the dreaded head gas kit. The water pump was changed about 20,000km ago and the expansion tank and cap were also changed recently. I think my problem might be down to the coolant not circulating properly. Recently after the coolant low light on the dash came on I checked the expansion tank immediately and the level was just below minimum so I did not add any coolant, once I restarted the engine the warning was no longer showing so I drove it a further 15km and checked the expansion tank again, this time the level was well above max. Could it be down to a faulty water pump or possibly air in the system? Any help on the this matter would be greater appreciated. Otherwise the car is driving perfect.