On my Q4, I need to keep topping up the Coolant. Strange because it doesn't drop an water, it's not mixing with the oil, there are no apparent leaks. The water temp is normal ish.. 90-95 & the main fan cuts in as usual. I believe I have a water cooled turbo, but that can't be normal... can it?
You may find that there is a leak. Maybe at the filler cap. When the engine is hot, the coolant is under pressure and may be being forced out as steam. That is why you'll never really see a leak.
The previous idea of using Radweld or simmilair is good. That will cure any small leaks in the system and prevent further small leaks. Costs about £4-£5.