thermostat?

The cooling system is a closed or sealed system and should not really require frequent topping up. I presume a shop has pressureized the cooling system and looked for a leak. Coolant leakes can sometimes be difficult to trace and may only leak as a car is warming up or leak when the engine has reached operating temperature. Coolant loss may be a sign of over heating, where does the temperature gauge read when you are driving under load and in traffic? Do the cooling fans come on when they are supposed to. Coolant loss may be a sign of a blow head gasket as well. If a repair shop has pressurized the cooling system and found no leak and can assure you the head gasket is OK perhaps try adding a product called BARSLEAK to the radiator. Follow the instructions don't add too much. It seaks out coolant leaks and fills the void where coolant can leak and seals the source of the leak. The liquid brown pellet type seems to work well. If you can't find that product Allumaseal powder works great as well.

Check the coolant bottle. The bottle will develop leaks underneath it. This will only be visible if you pressurize the system. If you do see evidence of leakage, 3 screws disconnect the bottle, turn it up and look at the bottom. You will most likely find a small crack and evidence of fluid leaking. Replace the coolant resivour, refill with 1 gallon coolant and let the engine warm up, then either go drive it or rev it up a few times to get the fluid flowing through the heater core and heat back to the car.

You may want to check your dipstick in your oil pan to see if you have any extra fluid on your dipstick or if the oil is discolored because your block may be cracked which is an internal leak.