One thing you can try - lick your fingertip and then wipe it on the windshield above one of the defroster vents - then taste it, if it is sweet, you have a tiny leak in your heater core.

Check around the reservoir, the radiator, and any hose connections - a small leak will dry, but leave a residue, often a white powder

If removal of the cap on the reservoir causes a release of pressure sound after a night, and your engine is stone cold - you can't have much of a leak.

How are you topping off the coolant?

I suggest that you only fill it to the MIN line when the engine is stone cold.

Check it every morning to see if it is going down. A pain in the neck, but it does not cost anything.

We had an '89 745 GLE that would not stay completely full. It always went down to the mid point between MAX and MIN between services,

My dealer would top it off at each service and a few days later out would go down to the mid point again.

Eventually I drew a line at the mid point and marked it "full".

The techs never topped it off again and it never lost coolant for the next 4 years

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.

I hope you are saving up for a replacement car. It could be a broken head gasket.

Volvos and other European cars have pressurized coolant systems, not just an overflow bottle. When the pressure gets too high, the cap has a builtin valve to release some of the excess.

I hope you are not just adding water, the boiling point at 212F is too low. Antifreeze 50/50 mix is required.
--
Keeping it running is better than buying new