low engine coolent , but no evidence of a leak on 2006 Volkswagen Passat

the low coolent level light comes on and dings, i check the coolent level and it is low, but there is no leaks anywhere and the engine oil doesn't look milky. but this continues to happen, about every week, iv'e putatleast two gallons of coolent in since this problem began, but still can't see any place it leaks.

Asked by for the 2006 Volkswagen Passat
its going out the exhaust you have a bad head gasket and its getting burnt through your engine