this is one of those things that you can't really diagnose over the internet, the system losing coolant like this could be from a pressure leak, or added pressure (head gasket). While I have not heard of common head gasket issues on these cars yet. Even a pin hole leak that takes the pressure from the system will allow it to boil over and out.

pressure raises the temperature it takes for the coolant to boil (boiling point)) so if you lower the pressure with a leak or open system it will lose coolant at a lower temperature.