I have noticed intermittent loss of anti-freeze. I lost around 5 liters antifreeze after driving 150 miles in 3 weeks. Well, loss pattern is very strange. I usually check coolant level before and after driving. I never noticed loss of antifreeze during journey, it looks it is long time after engine is shut off and it is not always.
Start by checking for leaks. You will need a pressure tester. It attaches to the radiator and you can pressurize the cooling system. Once there is pressure in the system, check to see if there are any leaks. Also, make sure no coolant is mixing with the engine oil. FInally, keep an eye on the exhaust system to see if there is white smoke coming from it. That would indicate a bad head gasket.