BBM -
Where did you read that? I would say that it is incorrect. The smell would not be that bad at first, but the longer the car sat in the hot sun, the worse it would get.
From a Death/Decomp cleanup service website:
http://www.cendecon.com/services/human-decomposition/decomp-cleanup
Something to know about doing a decomposition cleanup – there is more than meets the eye. Body fluids can and will seep into any porous surface. If a body decomposes on the 2nd floor of a building, chances are you will need to locate, remove and treat areas on the 1st floor, depending on how long the body was left.
Another thing to consider with a decomp cleanup -
the longer the body went undiscovered the worse the damage and smell. It’s pretty much a guarantee that all carpet, furniture, clothing and other belongings will retain the death odor until properly treated - even if these items were on the opposite end of the house.
ETA - As for the "Trace Evidence" sentence, well, Casey and the family had some time to clean up the trunk BEFORE they called LE. We'll never know what they did, probably.