I have two kids, now 6 and 8 so Im thankfully past the time of being able to forget them, or even if I somehow did, they would just get out of the car and be like "Dad, what the heck, we were still in the car".
I never forgot they were there, never left them in the car once alone, but one time I was supposed to drop them off at daycare when they were both really young, and I ended up driving about 5 minutes the wrong way (to go to work) with them in the car, before it hit me that I was taking them to work with me (turned around, dropped them off).
Im pretty sure (like 100% certain) I'd have noticed them though, and really don't understand how people forget their kids are in the car, I assume its mostly a 1 day change in routine that throws them off, but still...
SO, future parents of GAF, just put your water bottle / lunch bag / wallet / phone, something you need to take with you, in the back seat, so you HAVE to turn around and grab it.