Not all dogs are crotch-sniffing morons. Some of them are smart enough to know that the inside of a car is warmer than the outside even when it isn’t running.
In Washington one family thought they were the victims of a prankster who kept putting the same stray dog into their car so they setup a video camera to try and catch the joker in the act. What they discovered was that it was the dog itself that was opening the car door and then slipping inside.
It’s probably a good thing they found out before the dog figured out how to hotwire the car and take off for Florida.