I've seen the same thing plenty of times, as my garage regularly stays over 100F all day during the summer (South-facing doors with direct sunlight). The MyGreenVolt app will show temps for HV battery pack (as well as engine, transmission, and electric motor) and it's very rare that the HV pack would ever be more than 80F, even after hard driving and L2 charging. The Volt's HV pack thermal management system does a great job at keeping that pack between 60F and 80F.+1
Last summer, I was out in the garage doing stuff when the car A/C started by itself. It had been plugged in for about 26 hours, and long since finished charging. My guess at the time was battery cooling. The temperature was in the mid to high-90s.