Is it ever OK to leave a child alone in a car?

I'd had a relatively stress-free visit to Tesco yesterday. My daughter stayed with my husband at home and because my son had no-one to goad, he was well behaved. We were rushing a little as we came out because we'd bumped into friends inside and had a chat and were pushing it a bit to get home in time for lunch. As we walked out of the store I heard a car alarm going off. I didn't think anything of it. We walked past the disabled parking spaces and the car alarm got louder. I glanced to my right and was shocked to see a toddler in a car seat in the front of the car.

She was alone and she was distressed. Every time she moved the car alarm went off. She had her hands over her ears, tears streaming down her face and was crying uncontrollably. I stopped and looked around. There was no-one nearby. My son told me to get the girl out. It was my instinct too, but there was no way I could. He told me to call the police. I didn't want that to be my first response.

I stopped another shopper who was on her way in to Tesco and asked her to tell the staff that there was a child stuck in a car by herself crying. I was concerned because it was a warm day. Not hot, but 21 degrees according to my car, and the sun was shining on the car the child was in. You're not allowed to leave dogs in the car, so why would you be able to leave a child in the car with the windows shut.

Staff came outside and took the registration of the vehicle. They then went back in and started making announcements. By this point I had been waiting next to the car for around 15 minutes. There were two staff with me and the woman who had gone inside to tell them. After two store announcements, a woman came running out. When she got close enough, I heard her say "oh, has she woken up". The woman who was with me flew at the mother, shouting that she wasn't fit to be a mother and that she was going to contact social services. I was calmer and simply pointed out to the mother that this child had been absolutely distraught for at least 15 minutes and that every time she moved the car alarm had gone off which had made her scared.

At this point I walked away. Once I knew the child was out of the car, I didn't need to be there any more. We got into our car and my son was concerned that the woman hadn't been told off enough for having left the child. It's pretty hard to explain something to your child that you are struggling to understand yourself. I said that I thought she had left the child because they were asleep and didn't want to wake them up. My son thought that wasn't fair. He's right.

I began to question the whole thing on the way home as I will admit that I had been pretty shaken up by the experience. Is it ever OK to leave a child alone in a car? I know I have left my children, but only when I have paid for petrol or once or twice when I nipped into the corner shop. I could see the car at all times in both situations. I genuinely don't understand why you would leave a child in the car and go and do your weekly shop, or shop in a large store likely to have queues. Not being able to see your child is a definite no in my book. I have been in the situation where my children have fallen asleep just as we got to a supermarket and I know you're not meant to wake a sleeping baby, but surely you shouldn't just leave them? That poor child woke up alone in a car with a car alarm sounding. It can't be worth distressing a child like that just to let them sleep a little longer.

In the end, I decided to report it to the police because the more I thought about it, the more uneasy I felt about the whole situation. What are your thoughts? Is there any occasion when it's OK to leave your child in a car?

(the picture is my daughter sleeping on a coach, not the child in question)