Leaving animals alone in a vehicle in hot weather-- even briefly-- can be a real issue and it really depends on the vehicle, the animals, the area (crime and so on) and above all, just what kind of weather you're talking about. Ninety is one thing, 115 is quite another. Also, the color of the car is a factor in how hot it will get with the windows down. My cars are always white for that reason.
My little Escort wagon, crosser of many miles on I-40, had a key so loose that I could pull it out and leave it running. So I would do that and leave the dogs in the car for very short times.... such as running in to pee or buy a snack on the road, since I had no other human. One day we pulled into a motel parking lot and I ran inside to check in. Unfortunately I forgot to pull out the keys and.... worse luck... I had also not replaced my spare key in my purse after the oil change just before the trip. So it was in the car, too.
Ruh Roh! Fortunately the car kept running and the dogs stayed cool while I waited for AAA to come and open the door for me.
One thing that's always a good idea, though, is a spare key in your purse or wallet, whatever you always carry on you. Ninety-five percent of the time that spare key will be taken with, even if the purse or regular keys are forgotten. My spare wasn't there that one day, but has saved me many other times.