Crap......loss of coolant is a hole other can of worms.
As far as the heater core, coolant flows through there ALL the time, regardless of the temperature and airflow settings.
The blend door position determines how much air flows over the hot heater core, which controls the temperature.
I would top off the coolant, making SURE that you fill the radiator also (remove radiator cap and make sure.....squeeze the upper radiator hose if the coolant in the radiator is low....to push the air out of it.)
Then closely monitor the coolant level to see if coolant usage continues.....and look for any signs of leakage.
Hopefully there is nothing serious wrong.......remember, you have a waranty in force on the engine, so let the shop know if the coolant loss continues.
External leakage should leave white or green, etc (depending upon coolant type) stains or puddles under the vehicle.
I really am thinking away from an internal engine leakage, but who knows.