Mystery coolant leak on V70 T5, 1997

[cn90]

So, this is my theory about "normal" coolant loss...

My 1998 S70 GLT has 185K, with no coolant leak on the ground.
- Radiator was replaced by dealer at 140K, zero leak.
- Heater core is ebay Estonia brand, zero leak.
- Reservoir and cap is Volvo OEM , installed 2 years ago.
- Turbo coolant hoses replaced 2 years ago using Volvo OEM hoses.

Basically, the car is well-maintained, "nice and tight" cooling system.

Last time I sent oil for analysis, there was no glycol in there, so the head gasket is likely OK.

So, I wonder if vapor is released from the cap at every drive cycle, creating vapor loss...