Of course, she might just have popped it there for a 'second', intending to take it back to the trolley park when she'd finished what she was doing. That's what happened to me a couple of weeks ago, though it was my own car I left it behind, and didn't remember it was there until I reversed into it! I won't be mentioning that one to DH.
What a pity you don't recieve £5 for every odd person you come across Frank. You would be able to pay for the whole of gransnet to go for christmas dinner.
I was in Tesco's car park this morning sitting in my car waiting for my daughter to come out and a woman emptied her shopping in to her car and put the trolley in front of mine. I got out and put her trolley in front of her car when she was about to drive away. She was annoyed.