Thanks for the quick and helpful answers. It makes perfect sense and even explains why I have to refill coolant without seeing a leak, And in 3 days the water had much more time to intrude than with every-day-riding. Maybe that is also a reason why the clutch slips when the engine is cold and why the oil level differs (had to add quite a bit. Plus that I noticed some weeks ago that there was pressure in the oil tank (when unsrewing the dipstick...pffff!). Allright, good to know who the enemy is...so long, time to eat and have a beer.
Ciao : )