I wouldn't leave a multi frequency machine in a hot car. I lost more than one tuned search coil from my Whites V3i after first getting it. I am overly protective with my machines after that.
It's the resins which re-flow that kills a multi freq coil, and the inside of a car, in Texas sun, gets very hot. Maybe a single frequency unit is perfectly safe. I simply presumed all manufacturers had done environmental tests for heat.
I have begun using thermal bags for my coils during summer trips, with a frozen bottle of water inside. I will get thirsty anyway. Piece of mind.