Keep in mind that these dogs will “hit” on the scent of ANY dead body, not just the one that’s the subject of the case. There’s not a police department on the planet that isn’t capable of placing dead body scent anywhere they want to.
The mother is a doctor - not a huge step to believe that she could have had the scent of death on her clothes.
I know we don’t have great confidence in the authorities there, but:
“Police believe Madeleine died as a result of an accident and her body was hidden before being moved in the McCanns’ hire car.”
IOW, maybe not deliberate murder, but hiding the fact she died.
As to the dogs - I may be mistaken, but dogs are trained to track particular people. Could not this apply to cadaver dogs, too?