Might be the Auto Defog setting that is doing it, if you haven't turned it off yet. It is a setting located in the center stack settings, not the front/rear defrost button.
A lot of Gen 1 owners feel that the Auto Defog runs too often and just leave it turned off. However, in my Gen 2, I decided to leave it on, and I think it works better that the Gen 1 version to keep the widows from fogging up. It uses a sensor on the windshield to determine humidity and can turn on the A/C and/or heat automatically.
I see from your signature that our Volts are nearly twins. They were probably on the assembly line together.