I used to run into a local 7-eleven to get a slurpy for my older son and I, and I would leave my baby twins in their carseat, especially if they were asleep. I didn't have to worry about the heat, and I always parked where I could see them. We also live in a very safe neighborhood.
My kids are 8 & 10 now, and nothing happened to them.
I guess it's like this, if your going to be longer than a minute or two and you can actually eyeball your kid, they need to go in with you.
We've done that very same things many times before. I used to run in the store for a pack of cigarettes, it took all of 45 seconds. Two of them are in their 30's now. They survived.
Hell, I used to hold my newborn cradled in my left arm as I drove the car with my right. I burped 5 of my kids on my shoulder as I drove down the highway.