Volkswagen Passat water in oil not coming from engine coolant system. i.e. where from?

My VW passat is getting water in the oil raising the oil level dangerously high, however it isn't coming from the engine coolant system as the water level remains constant and the engine isn't overheating, my mechanic is scratching his head. Does anyone have any ideas about where its coming from or how its getting there, all suggestions gratefully received, many thanks