Read this article. Maybe it will help you understand how it happens:
http://articles.washingtonpost.com/2009-03-08/news/36840402_1_courtroom-tissue-class-trip
The reason leaving your purse or something else in the car can help is because some people (not me, I leave my purse in the car ALL the time, sometimes for days on end. My cell phone too, actually) have things in their purse they cannot go long without.
I recommend that parents think really hard about something that, if they were to leave it in the car, they would be FORCED to go back to get it in a short period of time. Leave that in the back seat, every time, whether the child is with you or not. That way you have to open your back door and retrieve it every time you get out of the car.
That item will vary from one person to the next. If you work in an office, it might be the keycard that gets you into the building. It might be the little USB drive that stores your encrypted network passwords. It might be your laptop computer. It
might be your cell phone. It might be the keys to your filing cabinet. It might be your briefcase. Heck, it might be your chap stick if you're one of those people who can't survive 30 minutes without refreshing their lips. Or your water bottle. Or...
As a stay at home parent, frankly, my kid is the thing I would notice was missing right away if I didn't have her with me. I haven't figured out yet what that "thing" is for me now that I'm not working. I had several items that would have worked, had I been aware of the danger, when I was a working mom. The problem isn't that you forget your kid all day. The problem is that you forget your kid *for a moment* at a critical time and then you go about the rest of your normal day without having realized you had that lapse at 7:30 this morning where you didn't turn off to drop the child at day care. Most people can't even remember what they had for breakfast this morning. Your brain is incapable of differentiating "this morning" from every other identical morning where you dropped your kid off at the sitter's house. You can
remember dropping the child off, even though it didn't happen, and not realize that the memory is manufactured out of the merging of 399 other identical times when you actually
did drop the child off.
After reading the article, it came to my attention that one additional risk factor which we would do well to avoid is talking or texting while driving.