Let me add another thought...
The body didn't have to be in the trunk for days for the smell of decomp. As a first responder, I've pulled dead people from vehicles before. Don't read any further if you have a weak stomach.
In one case, the deceased's spinal fluid leaked from his nose and pooled on the floorboard of his van. We removed him immediately after the collision, after the coroner's declaration of death, but I would imagine that the smell in that van, after sitting out in the hot sun on the wrecker lot, would have been unpleasant, to say the least.
OK...I'll not gross y'all out with any more stories like that...well, except for this one. I've also smelled burned bodies, and, as for the most offensive, most memorable smells, this one is only second to decomp. If Casey had burned Caylee's body, anyone within a mile would have smelled it.