There is still a bit of ambiguity about TMS on unplugged Volts in hot weather. This is what I've been able to glean.
1. A Volt with high SOC will use the TMS as needed to protect the battery. This shouldn't be hard. The sun's heat has to make it through the car and the extra insulation around the battery. The TMS has direct access to the battery. Guess who has the edge?
2. A Volt with relatively low SOC will not use the TMS to protect the battery but batteries with low SOC are less vulnerable to permanent degradation and the battery insulation is still there.
3. The white Volt car cover is quite effective in protecting the battery. I've parked all day in 100 degree weather with the Volt car cover on and the interior was not very hot. Since the battery has a lot of insulation, the battery should be safe.