I always wondered if it was possible that the scent in the car was transferred from something else. For example, if Madeleine died in her bed, then her bedding at some point ended up in the car (then was removed again before the dog search). Would that possibly transfer a scent?
I still also stand by my opinion that 'death scent' in the flat doesn't necessarily implicate the parents, but their firm denial that Madeleine came to harm in the flat is very strange.
If madeleine came to harm in the flat then chances are she is now dead, if you were her parents would you do anything you could to believe she's alive? People do anything they can to believe a loved one is OK.
I see your point about the scent, but the dog never alerted in the boot of the car just to the fob and passenger door which they found the fob in ( I linked to this earlier). I think if scent from a dead body transferred in this manner there would be alerts everywhere from people who handled the body.