I've been to one once. Member of public, absolutely fuming about a dog left in a car park in a field, at a car boot, in a Range Rover.
When we got there the lady was slightly inebriated (had been having a bbq) and took us to a German Shepherd, which was indeed locked in a Range Rover, in about 28 degree heat.
Thing was, the dog was fine, as the car was running with the air con on. It was still locked though - you could see the keys in the ignition. They had obviously felt it was safe to risk leaving it as it was 'blocked' in by cars front and back. *
I couldn't get through to the lady that the dog in question was probably a lot more comfortable than us!
*yes I'm aware of the offence of 'quitting'. No I didn't do anything about it. I also agree with anyone who thinks it wasn't a clever idea to leave the keys in a shiny Range Rover, though perhaps the dog was a guard dog - I wouldn't have tried it!