We have a 2007 Pontiac G6 GT and it is constantly using coolant. Every so often you can find a large size puddle under the passenger front tire. While the care is at idle it doesn’t run hot. While driving it reaches 225, the air is cold, and the heater blows warm air. You can stop and hear the fans running. There is some discussion about GM’s problem with the orange coolant. Not sure where to proceede. Water pump, intake gasket, or head gasket! A leaking gasket would leak heavily all the time. Car has 50K miles and saw the problem begining at 30+K miles. GM pressure tested then and found now problems.
You may have a failing water pump, at the bearing/seal. And it may be leaking only when under pressure (operating temp) and when the pump is spinning (engine operating). Since the pressure test is done with the engine off and cooler, it may not be showing the leak.