A couple of points come to mind here:
1. If the police thought that there were serious concerns (beyond an isolated incident) about a child's welfare they might well have alerted social services, so it's possible that they actually did more than 'administer strong words of advice' ;
2. It's possible that the mother might have just nipped into Next to, say, return a garment (expecting to be away from the car for no more than a few minutes) and then have got distracted from what she was doing. Even the most loving and caring parents can make such mistakes.
For example, a friend of mine was the most dedicated teacher I've ever known and loved by all of his pupils. He was also a loving and caring parent, as was his wife. After shopping at Asda one day, they loaded up the car and drove home. It was only when they got there that they noticed that their 2yo son wasn't in the car! They'd left him, sitting in his pushchair, in the Asda car park! (When they got back, having broken the speed limit all of the way, he was happily chatting to passers-by!).
So perhaps the mother just made a one-off mistake (like my friends did) and, just like them, was amazed at how she could have let it happen? We're all human!