You did the right thing in staying OP but I agree that I still would have called the police afterwards.
Maybe the fact the child was so calm for the duration was because they were used to it because they’ve been left alone in cars for quite some time.
There was an incident near me once where a mother left her child in the car (he was about 7 years old) in a car park whilst she went into a nearby shop. Whilst there she had an issue with her heart and she collapsed, didn’t regain consciousness and was blue lighted to hospital without anybody having any idea there was a young child on its own in a car.
It wasn’t until her husband arrived at the hospital and asked where his son was that it all unravelled. By the time the police found the car the boy had been alone in there for almost 3 hours. It was a very scary story and one that’s never left me.
I have a 7 year old and I would never leave him in a car alone, never mind a younger child.
And as a previous poster mentioned above, it wasn’t that long ago in my area where a car was stolen with two young children in the back whose parent had left them in their alone whilst they’d popped into a shop.
The risks of these things happening are very low - but children are too precious to take those risks with.
You did the right thing to not address the mother as I’m sure that wouldn’t have done down well, but I still would have reported her so the police would speak to the mother and hopefully stop her from doing it again.