2003 Chevy Tahoe

Check all your hoses, and for cracks in the overflow tank (aka 'degas bottle'), check the radiator cap as well, those should be replaced every few years anyhow, they can lose the ability to maintain pressure allowing coolant to drain out via overflow. Also check bypass tube at waterpump/thermostat housing. Finally make sure you have completely bled all air out of the system. There's also a remote chance your head gasket is partially blown causing the cooling system to be over pressurized, forcing it out through overflow. Block Tester can check coolant for combustion gases, or run a compression test.

Heater hose or the heater core the hose goes to.