2012 Prius three

Hello all, I have 2012 Prius with engine coolant loss. I have logged over 200k. Always done services at Toyota dealer. Recently I would add coolant a bit full and then a week or two later warning light comes up especially when going uphill on freeway. Then I would turn blast the heater to full and the light would go away. The coolant level dropped again and then I would add some more coolant. This has been going on for few months. My one gallon coolant bottle is almost empty now. I don't see any coolant leak on my driveway nor anywhere on the vehicle that I can see. I just changed the motor oil for the first time on my own and the used engine oil looked normal. Could this still be bad head gasket problem? My Prius runs perfectly otherwise. Any other possibilities? Thanks in advance.