I've also heard of people claim that they left rounds in the car and they started to "melt". It sounds like BS to me, but I figured I'd ask anyways just to be sure.
Lead melts at 621.5 F. At that temperature, all the door panels would have pooled on the floor.
There's a Ruger LCP in the door pocket of my car in central Louisiana. It's been there a couple of years in the heat of our summers, and it absolutely hasn't melted.