Oh, I am so sorry! That is so scary to learn your toddler was left in a potentially dangerous situation unattended! I am glad she is OK!
This is serious and from your description of his behavior it sounds like you will get nowhere trying to talk with him about it. Do you have a lawyer? If so, I would call him/her and ask how to proceed.
Are you documenting these episodes?
This, absolutely. You want to know exactly how this effects the legal situation before you bring it up with ex.
I'm with you, by the way, in that I would be totally flipping out over this. Acceptable amounts of time to leave a toddler alone in a car have limitations like "long enough to return the shopping cart." At 21 months, my kids could both unstrap themselves, what if she'd decided to try playing with the emergency brake? And how terrifying for a child to wake up alone in a car and just have to wait, not knowing what happened, until somebody came and got her.
I think the risk that someone would break in and kidnap a child is pretty low, but here's something far more likely - a concerned neighbor spots the toddler all alone in the car, and calls 911, standing by the car until emergency responders arrive. That's what I would do. Emergency responders don't waste time jimmying the locks, they just break the window least likely to shower the child in broken glass. They run the tags on the car at leisure after the child has been transported to the ER for evaluation. Your ex comes to check on the kid between episodes of Friendship is Magic and discovers that his truck is wrecked, and the police not only have a warrant for his arrest, they are not exactly forthcoming with the location of the child.
The baby could have been sleeping a long time...or just a few minutes. Also, my 10 year old doesn't have a good concept of time, so he would have no clue how long something went on for, if asked. It is likely that the baby was there a very long time. Perhaps the TV was turned on a half hour after they got home, or she watched three shows, not two, before they went to get her sister from the car. Also, 80 degrees is pretty hot in a closed car, even in the shade. I wouldn't be so sure there are only two spots either, and who knows if all entrances and exits were locked, or how many people had access...locked or not.
I had an interesting experience once when I left my daughter in my car. She was a toddler, but looked young because she had no hair. She didn't know how to unbuckle herself. I went to return a shopping cart and drop a book in the library book drop, right next to the grocery store, same plaza, same sidewalk. I calculate that this whole process took less than 2 minutes. As I'm heading back to my car I see a woman standing there....she looks at me and screams "IS THIS YOUR CAR!!! and goes on and on about how dare I leave a baby in a car...I started to explain I only went to return my cart and she just kept screaming at me, and getting in my face, telling me she was going to call the police, etc. I just got in my car and left because I was so embarrassed. As I was driving down the road a couple of minutes later, a cop car started following me. I pulled into a parking lot, and he pulled in too! He kept going, but I think he may have been following me. I can't help but wonder if she called the cops or knew a cop in the area. In her mind, leaving a child alone for even a minute to return a shopping cart was grounds to call the police. She must have seen me buckling up my daughter, locking the car and walking away with the cart, thinking I was going shopping. She probably called the cops immediately. While I think this was a bit "overkill", I also see her point. It was a good wake up call for me. Something tells me if a woman did this to your husband, especially if he was only going to return a cart, he would be pretty irate.
I would call your town's non-emergency police line and ask them about the legality, etc of that. Then ask them to put it in writing and send it to you or your ex (so you can show him).
So I found out it is considered neglect. My lawyer says that the police would be called and CPS breathing down our necks. He was pretty adamant that the issue needs to be addressed and was very concerned about his judgement. I ended up not confronting x directly....but indirectly in a way that hinted I knew, but indicated not to ever do that again. It will be discussed in mediation. I'm editing/deleting the first part of this post for security/privacy reasons. Thanks everyone for your advice!