Thinking about it logically though, for the dogs' marks as presented to all be accurate it would presumably mean that the McCanns killed the child, waited long enough for the cadaver scent to transfer to the surroundings, hurriedly hid the body somewhere nearby in a location so excellent that days of intensive searching didn't find it, popped off for dinner, drew attention to her absence, then waited until they were 25 days into one of one of the most high profile missing person searches in history to move the body from its already excellent hiding place to another site so equally excellent that a further decade of said missing person search never found even a trace of it? All while in a foreign country they have little local knowledge of, without leaving hard forensic evidence behind and without any of the CCTV, eye-witnesses, journalists or police around them noticing?
Alternatively, sniffer dogs (who aren't exactly infallible, whose evidence is inadmissible in court without forensic confirmation in many jurisdictions and who can be misled by even excellent handlers) searching for indicators of forensic evidence among a mix of personal and rental property might not have been entirely accurate? Which (if even some of their marks were wrong or misleading) would point away from the parents once more. Also, I think I'm right in saying only one dog marked the boot of the car, the other marked elsewhere.
Whatever the unknowns of this case, I would be very sure the McCann parents weren't wandering around with Madeline McCann's dead body in the back of their rental car nearly a month into a global manhunt in which they were under the full glare of police suspicion and global media scrutiny. In which case any material detected was (logically) the sort of thing that could be transferred after a month and then found a several months later. Which (logically) also makes it too transferable to on its own tie the McCann parents to the McCann kid's murder.