Even if he didn't notice at noon, he would have noticed at 4:15 when he got in his car... and NOT have waited until a couple miles down the road. Considering LE said the car stank to high heaven.
That one guy did, that's true.
I'm looking at this in its totality. This is a dad who by all accounts is a wonderful guy, there don't seem to be any issues with him or his family, and it seems clear to me he couldn't have seen the baby in the back seat at lunch.
Shortly after driving off in his car, he clearly noticed something was VERY wrong and pulled off the road into the mall parking lot.
It's surprising to me sometimes what people do not notice. It's surprising to me what people can forget and be unaware of.
But to me, the other scenarios are more implausible. That this loving father purposely killed his child, or that he went to the car and noticed his child was dead and then went back to work for 4 hours and staged that he THEN found him dead (as opposed to finding him dead at noon which would be fully as incriminating - so working for 4 hours and then staging the exact same outcome doesn't make sense).
Those scenarios are MORE implausible than the one where he bizarrely forgot his child (in one of the major scenarios of kidsandcars, adding a stop to his routine) and also didn't see him in the back seat facing backwards at noon, and then it took him short while to smell the smell when he drove away at four.
When I have to consider what seems the most plausible, accidentally forgetting him, not noticing him at noon and then driving off and not noticing a smell for a minute or two is the most plausible.
You have to work too hard - in my opinion - to make the other situations rational.