I can't imagine EVER, EVER forgetting my son is in the car. (Not at any age... in fact, when he was a really tiny infant even less so, because my thought of "where is he" was at the very forefront of my mind practically 24/7) If I put him in the car as an infant, there's no possible way I'd forget he was there.
I guess I could somehow see it if a spouse put the kid in the car and (for whatever incredibly stupid reason) didn't tell the driver. When babies are really small, they ride with the child seat facing backward and you often can't just look back and see that the kid is physically there. If the baby was a super-quiet car rider (like my son was) then the combination could make for the perfect (in the worst way) storm.
But the damning thing in this case: looking up info on death in a hot car. What kind of sick fuck does that, and then it just "happens?" It sounds like the most wretched type of asshole planned this.
If so, I don't want him to fry. I hope he spends life in prison with all the hardcore criminals in there knowing "this is the asshole who fried his kid in the car"... not that it'll be all that long a life.