I don't know. If the smell was HORRIBLE - that's one thing. I'm very curious to hear what others said about the odor at the scene - I'm sure the car was taken to forensics and gone over. If I open the car up and a FOUL odor whooshes out at me, I would search it before getting in.
If I got in and the car smelled funky, and as I drove more it smelled funkier, I think I'd first imagine some awful thing was in the atmosphere. I'd turn the AC to "recirc" and maybe roll down a window to see if the smell was coming from outside.
I guess the basis of this is that I don't believe a baby would smell all THAT foul after such a short period. A cop saying it "stunk to high heaven" might indicate he could detect an odor he knew - human decomposition - in that car. It may not turn out that the smell was so awful as the media says.
Coroners wouldn't be able to do their jobs if little babies like that stunk to high heaven after 4 hours, IMHO.