Would you leave a 5yo in a car alone?

[Ellerehj]

My ex partner left my 5yo DS in the car alone while he went into a shop (for Non essentials I might add) Long enough that he was crying when he returned and DS has mentioned it to me a few times and it seems it upset him. He doesn't have a great concept of time but he said he think his dad was gone around 20 minutes.
Do you think this is okay? I was completely outraged. Am I right to feel this way?

20 mins is too long imo but I often leave my children in the car alone for 5 mins or so to pop in a shop or to drop one of them at an activity. The difference is that they know where I am, that I won't be long and they don't mind. I've never come back to find they are even slightly bothered by me being gone.

No way. I can't lock the doors in our cars without turning the alarm so they could merrily let themselves out. That is properly shit parenting.