I also don't think it would take long at all for staff at a petrol station to notice there was a child in the car. I park right next to the entrance, which means everyone would walk past my car to go in. They also have cameras on every pump. Usually when I walk in the attendant has already rung up my bill, as they tend to notice which car you have come from.
Plus if I was going to fall/ hit my head/ collapse then I would rather do that when DD was NOT in my arms!
Everyone weighs up risk differently.