Well, it could just be that the level has dropped slowly, over the life of the car, and this has never been topped up. Not good in itself, but maybe acceptable. Not sure, but the GTD I had before my GTI did not loose any coolant in the 2 years I had it. Otherwise, it could be that a leak has developed, due to the car being stood for a while, maybe through a pump seal, or even something as simple as the expansion tank cap seal drying out. Worst scenario would be a major engine problem, but that does not seem to be common with these engines.
My point is that you need to be sure, as soon as possible, that you do not have an ongoing problem with this car. I hope you get the right answers, the car is properly fixed and you go on to enjoy it. GTDs are awesome machines!
