I am constantly adding coolant to my 2000 S70. About a gallon every three days. I have replaced the heater core and the radiator. I have check the heater core again and it is not leaking or is the radiator. The car is not overheating. There are no apparant leaks that I can see. No puddles under the car. No water in the oil. The coolant is going somewhere and it is driving me crazy not being able to find where. Anyone have any ideas?

 

Do you notice any smell of coolant after you park the car and open up the hood? It could be leaking from a hose or water pump and evaporating before you notice it.
If not, then I'd say your head gasket is suspect and coolant is leaking into the combusion chamber and burning off. So do you notice white smoke coming out the tailpipe?
You might want to perform a compression check of all the cylinders.