Hello pretty sure the water pump is on its, way out due to the last few weeks coolant loss in expansion tank. This is variable. Car is booked in for timing belt and water pump next week needs doing anyway. Question is what is the best make of pump and what to avoid, due to so many VAG group pump failures. 

https://www.blauparts.com/blog/vw-water-pump-replacement-information.html

Didnt know there was a problem this may help.

Thanks so hard to find problem drove 60 miles the other day no issues came out next morning expansion tank empty and no leaks visible under car or engine bay. 

So woke up this morning to this. Expansion tank fully drained.Booked in Monday at least you can see the leak. First time it has been obvious. 

Good luck let hope its clear cut and traced to the water pump.

Problem was the water pump, changed along with cambelt. Looking back to last change water pump only lasted 32000 miles pretty poor compared to other makes I have owned.