Get a “cooling system pressure test” performed by your mechanic. This will help locate where the coolant is going, hopefully an external leak and not an internal engine leak.
This is a simple and cheap test that can usually be peformed while you wait. When the mechanic pumps air into your radiator it will push out the coolant/water from the leaking hole and make finding the leak easier. Sometimes I have had to leave the pressure tester hooked up to the radiator for a few minutes for the leak to show up. Make sure you check inside the vehicle on the passenger side underdash area where the heater core is located. When the heater core leaks coolant it will run down under the carpet on the passenger floor board. Good luck!