You need to have it pressure checked and it should be done by someone with

Volvo experience, dealer. It should hold pressure and maintain level while under pressure. It's either leaking externally, possibly to a place not so obvious like your heater core leaking into the matting below your carpet. That can hold ALOT of coolant before you see it or notice a smell or damp carpet.

Another possibilty is an internal leak. I've seen one rare S70 engine problem with a crack in between two cylinders where the head gasket sits and that caused a slow leak, didn't steam out the exhaust either but I found the leak after removing the plugs under pressure and saw coolant in the cylinders. The cure was a new engine, warranty.