So it seems that I have started to loose coolant fluid from my 2000 v40....

I continue to top up the reserve-reservoir to the back of the engine bay, nearest the passenger side firewall.

I do not see any wetness/puddle under the car when I go back out to start it after a drive or overnight parking.

Over time though... I seem to loose coolant, because the heating system inside will loose effective heat capacity, and I'll begin to have the heat-gauge start to climb up on longer trips.

When I go to re-top the fill reservoir... I notice that the lid makes the distinct sound of being under pressure, with gas escaping.... Is this proper or normal? I've never owned a car where this is the case... and it will sometimes be under pressure after a whole night of being parked and fully cooled-down, so it not just from heat build-up.

Shouldn't the coolant system have some sort of venting to prevent pressure build-up?

Any insight on this out there, much thanked....