, radiator 1999 Monte Carlo

If it's not leaking then it can be going out the tailpipe via a leaking head or intake gasket, or coolant is dumping out overflow bottle during use, or the heater core is leaking and coolant is again dumping to the ground via heater box drain. You might have other leaks that only appear when cooling system is pressurized. If you want to know for sure, find a mechanic that run a cooling system pressure check. If there is a leak that will find it.

Small leaks can go unnoticed. I would use Alumaseal to see if the coolant loss stops.