The first comment is that there MAY be a special fill procedure for your car. If you are not observing the fill procedure, you may not be putting enough coolant in, and OF COURSE the car will drain the tank, to fill the cooling loop, once the thermostat opens.
The second comment is that this sounds as though the car is burning coolant. Think "blown head gasket". Start by taking a look at the engine oil. Your shop may be able to tell something with an exhaust analyzer.
I had a problem with a 1990 Chevy Lumina 4-door sedan gradually draining the coolant tank. Newspapers under the car revealed a leak SOMEWHERE, but I couldn't see where. I finally got lucky, and saw a leaking gasket at the bypass union, that turned out to be a worn union. (That repair was what triggered my search for a replacement vehicle: getting a replacement part was a lot harder than it should have been.)