Meh. At 8, he is old enough to roll down the window if he is too hot, or get himself a drink (if he had one...) and basically take care of himself. I know when I was that age, I usually preferred to hang out in the car and read while everyone else was grocery shopping. We had a HUGE family, so grocery shopping usually took at least an hour to two. I was perfectly content to chill out in the car the entire time. Heck, I was staying home unsupervised for up to 5 hours when I was 8! lol. I think that things like that depend on the maturity of the individual child.
The only thing I would have an issue with is the type of neighborhood you were in. If it's really that bad, I don't think I would have been willing to leave my kid alone. Of course, he was in a car that is lock-able (when I sat in the car, it was always locked so no one could just climb in...) and at his age he most likely knows how to scream and draw attention to himself (not that it would do much good in such a big parking lot...). He could have had a cell phone to use if he needed something as well.
I would have done like you, and let that officer know. Not because I think that it's wrong for an older child to be left in the car, but because the neighborhood is one where extra supervision would be beneficial.