This may sound really smart-alec like, but is not intended to be... do you have split controls for your rear cabin on the back of the center console?
I raise this on the basis it happened to me just last month. My kid got cold and used the rear controller to turn the hot/cold knob to warm. I didnt know/realize it. I thought my AC was needing recharging for 2 weeks.
I have had that happen to me so many times its not even funny! The other one that gets me is my buddies will turn on the heated seats while I'm not looking.