Yikes! I would like to say I would have done the same thing, but I was in a simliar situation once. It wasn't cold out, or hot. Mild weather. My mother, DD and I were at a little strip mall. DD and mom were inside and I was standing outside; a woman pulled up, attempted to get a screaming preschool aged boy out of the car, got frustrated and left him while she went inside a pizza place. She could not have seen him from inside the store. I figured she would be in and out, maybe picking up and order she'd already placed but she was gone quite awhile. The child was screaming the whole time.
I just froze. I'd never seen anything like that before, and I'm still kicking myself for not acting on it. My mom came out of the store, saw what was going on and reached for her phone to call 911 when the mother came out of the store. My mom yelled at her for leaving her child in the car alone and the mother said, "Haven't you ever seen a tantrum before?" or something like that.
So it's a slightly different situation, maybe not as bad, but still. I would never, never, never leave me child in the car. I'm not judging the women in here who've said they've left them in an emergency but I really cannot see myself doing this. It's something I actually have nightmares about quite a bit.