I don't drive so can't give a real insight to this as I'm not sure if I'd leave Ben when paying for petrol, but I'd certainly never leave him for any other reason, I wouldn't leave him full stop, like Suzi said why take them if you're not going to involve them? The thought if someone carjacking the car with him in it (if I had one!!) is a big fear of mine, as is burglary etc!
When I worked in a shop, about 6 months ago a lady came in and was paying for something, she kept checking out the window and I noticed her car with a toddler (no older than 2) in his car seat, with all the windows wound down. She kept waving so he knew she was there. I felt sick just watching but then she wanted to look at something in a part of the shop where you couldn't see out the front, she just went off for 3-5 minutes with my manager. I went right to the front if the shop and watched that car like a hawk, all it needed was someone to put thier hand through, unlock the door and hot wire it.