First I would clean everything around anything coolant related. Add some dye and do a pressure test.this will determine if there is an external leaks. Then do a compression test to check for a bad head gasket. If you still haven’t found the problem, drive it for awhile and try and maneuver something up the air conditioning condensate drain and check for dye in the moisture you pull from the tube. If there is dye,
The heater core is leaking.