I honestly don't have a very good answer. But I have a couple thoughts on it...
-My brother and I were left alone ALOT at home and in the car, and never thought it was wrong....That said, I remember a time when I was about 3 and my brother was about 8 and we were left alone in the car at a grocery store. The driveway was kind of a slope, like a hill, and we were parked at the top and one of us (can't remember who-fuzzy memory) hit the parking brake and we started to roll, and all I remember is being scared and then my grandfather coming out and physically standing in front of the car and stopping it. My mother thought we wouldn't be able to unlock the parking break but somehow we managed...this has always made me leery of the idea of leaving a child in a car alone, especially on a hill!
-I really don't have much experience at this point as my children are my eight year old stepson and a 7 month old. I don't leave the ss alone in the car that I can think of, as he is easy to get in and out of the car and likes to go in with me. So if I go to a gas pump and have to pay inside, he just comes with me. Of course I do leave him in the car alone while I pump the gas...he doesn't come stand by me. I've never been in the position where it's just been me and the baby in the car, so I've never had to leave her alone, my mom or partner is always in the car with me. I wouldn't leave the 8 year old and the baby in there alone either, I'd be too worried that she would start crying and he wouldn't be able to sooth her.
-Also, in our apartment building, we are in a large apt. complex on the 10th floor where each floor has it's own hallway. Yesterday, the baby was asleep in the stroller, the I struggled to get out the doorway without having the door hit it and waking her up or by hitting the doorframe and waking her up. Once I got her into the hallway, I remembered something I left in the apt. So I either had to struggle to get her back in or leave her in the hallway while I ran back in. Well what I wanted was not in it's place it usually is, so I had to search for it. By the time I got back to her, I was panicked. What if she had started crying? What if someone came by the hallway and grabbed her? What if someone across or down the hall had taken her into their apts? No, I can't ever imagine myself leaving her!