I have to be devil's advocate--because I understand (on some level!) how people don't want to know the truth. The truth in this case was wayyy more than they could take. So George knew that the smell was awful, but when he opened the trunk--whew! he knew that no one was there. What a relief! Then he probably thinks something along the lines of GAWD did a cat crawl up in the wheel well and die?? (I know that doesn't sound feasible but we are talking about someone who wanted desperately to be able to think about a live Caylee, not a dead Caylee). I did have a snake crawl up into my car when I lived in Fla. It died under my back seat--believe me it was horrible and I had no idea what it was. If, during that time, I also had a missing daughter/granddaughter I am NOT so sure that my mind would automatically "know" the smell had been one of them. (I know it crossed GA's mind, he admitted that--but he didn't want to think it and replaced it with whatever bullcrap he could accept).
Merge all this thinking with the Anthony's innate feelings of superiority (they do think all the authorities in Fla aren't very smart, or aren't capable) and we get the hair brush thing. BECAUSE these hapless grandparents wanted a different outcome from the truth. Is what they did wrong? Yes. Is it criminal? Yes...but I bet LE thinks oh these poor old wierdos..they are so distraught, they are incapable of handling this.
In an early interview the FBI tries to tell CA, they try to tell GA..but they have probably seen plenty of this type of delusion in their jobs. They are sensitive to it. I bet someone has told the A's stay out of all of this from here on out, or the sympathy will be completely put aside and they could get charged. Perhaps that is why they have stopped pounding their chests to say that SODDI.