2000 grand prix losing antifreeze

My 2000 grand Prix motor is very clean and no grease or dirt to be found on it,my low antifreeze lite keeps coming on every few weeks and I need to add just under a half gallon every time and I can't see where its leaking out,there's no marks left underneath the car and I even checked the oil for antifreeze. WHERES IT ALL GOING....

My 2000 grand Prix motor is very clean and no grease or dirt to be found on it,my low antifreeze lite keeps coming on every few weeks and I need to add just under a half gallon every time and I can't see where its leaking out,there's no marks left underneath the car and I even checked the oil for antifreeze. WHERES IT ALL GOING....

If you are adding 1/2 gallon every few weeks, it should be pretty obvious. Have you tried a cooling system pressure tester? The usual culprits on those are the intake plenum gasket, coolant elbows, water pump and heater core...