Wild guess: Cars generate a lot of heat, most of the power in an internal combustion engine is lost as heat (they're something like 30% efficient). The engine b

Wild guess: Cars generate a lot of heat, most of the power in an internal combustion engine is lost as heat (they're something like 30% efficient). The engine block and body absorb the heat when operating, and when the cars are shut off (not to mention running when parking/exiting), the heat radiates out into the air.