It is either the HBV valve or the oill cooler. Mine had a small leakage in one of the pipe fittings in the cooler, had to replace the whole thing (no ttoo cheap), the HBV is affordable and easier to change. Not too many other possibilities. Also sent you a pm in Finnish...
There is always a chance of a small leak from a hose or from the radiator drain tap, but as said above by far the most likely reasons for having to top-up coolant on the V6 are Heater Bypass Valve and the Oil Cooler.
Also, you need to make sure that you are using Vauxhall/Opel Red Antifreeze (not green) with 50/50 concentration, and that it is replaced often (1-2 years). Officially its is 2 years for the green coolant and 5 years for the red coolant, but go for the red coolant and replace maximum every 2 years. The oil cooler is very sensitive to corrosion, apparently even the newer stainless steel ones..