Sounds like you're losing coolant into the exhaust. A pressure test will help you find your leak. First attach a pressure tester to the radiator tank neck and pump it up to 10 to 12 psi. If the pressure drops you have a leak somewhere. If there is a leak but you don't see coolant leaking anywhere, then either the heat exchanger, the coolant tank, or the head gasket is leaking. Remove the hose from the heat exchanger to the mixing elbow and pressure test again. If coolant runs out of the heat exchanger hose, the heat exchanger is leaking and must be replaced or repaired. Otherwise the tank is cracked and coolant is leaking into the exhaust manifold. If the engine is running OK and the oil level is stable (it's not "making oil") then it's unlikely the head gasket is leaking.