Car overheating and losing coolant

It's possible to lose some coolant thru a head gasket that doesn't show up in the oil or out the exhaust. A pressure test on the cylinders will reveal that type of leak. Another possibility is your coolant loss is from the overflow hose to the ground while your driving, overheating is causing coolant to overflow to the reservoir then from there to the ground. You may want to check the water pump, if the impeller fins are worn to nothing then it's just not pumping coolant sufficiently. Check the pressure in the rad hoses and heater hoses with the engine running. A new thermostat and rad cap is always a good place to start as well, rad caps should be replaced every few years anyhow, a bad one that can't maintain proper pressure may be letting coolant overflow to the reservoir when it shouldn't.