If my daughter was talking to my wife frequently, telling her all the things my grandaughter had been doing, I don't think I would believe her body had been in the trunk of the car I smelled decomp. in if I didn't see it. I wouldn't want to believe it.
Frankly, if I were a person who wanted to protect my daughter way beyond the point of legality or morality, I might think, there was the body of SOMEONE ELSE in the car, who was not Caylee, and I should talk to Casey, or have my wife talk to Casey, before I called the police.