Our Volvos' cooling systems don't have the traditional reserve reservoir. The whole system, including the reserve tank, remains pressurized.

My first reaction is to ask if the carpets are wet, is the heater core leaking? But the fact that the system remains pressurized overnight suggests that there's no such leaks. Still, check to see if it's leaking into the cabin, which might explain why you're not seeing it leak onto the ground. That's a much better outcome than a head gasket leak.

How much coolant loss are we talking about? Top off the reservoir once a day, week, month? How low does it have to be for the temp gauge to start to rise, is it only after you lose the heater system (down quite a bit a that point)? Does the coolant smell like exhaust or have tiny droplets of oil in it?

Please do stay on top the coolant level. If it gets so low that you lose heat, you're getting dangerously close to cooking the engine. And yes, antifreeze is best and necessary if there are freezing concerns. However straight tap water is better than nothing in a pinch and won't do any real harm as long as it's not used long term.