Radiator in car overheated?

with the engine cold, remove the cap and add a mixture of coolant and water. once it is full at that point, start the engine and let it warm up a bit. the coolant level should drop in the radiator once it is circulating. if you are loosing coolant like that, i would feel the carpet under the dash on the passenger side to start with. if that is wet, then the heater core is at fault. if that is dry, then look under the vehicle and see if you dont see a wet spot there, if yes, then the water pump or a hose could be the issue. of nothing is wet and the level does not drop, then the cap could have let it evaporate and leave that way.