I've done some more research.
I'm not wedded to any particular theory, but at the moment I'm interested in the 'exited the apartment via the front door' theory.
The question is what happened next? Are the tracker dogs giving any information, or is it irrelevant noise?
My understanding of the facts is that the dogs were used twice: 4/5/2007 23:00 and 8/5/2007 23:45
I've found some scientific research, which may be helpful (or not!)
https://k9sontrack.files.wordpress.com/2010/01/how-many-footsteps-to-determine-track-direction.pdfThere are two types of scenting: air scenting and contact scenting.
Air scenting appears to require the presence of the 'target', so the dogs were unlikely to be following an air scent because Madeleine was no longer in the area.
There does not appear to be scientific support for the scent arising as a result of Madeleine being carried by a potential abductor.
The second method is contact scenting.
This appears to require actual contact with the ground:
snip:
Furthermore, when individual odours were prevented
from being deposited through covering of the shoe
(Romanes, 1887) or by the wearing of a whole body suit
(Pearsall and Verbruggen, 1982) dogs were unable to follow
the trail. However, others have observed that dogs were unable
to track a trail of body odour cues, and could only follow
a trail when there had been ground contact (Budgett, 1933).I believe it is generally accepted that Madeleine was barefoot, because her shoes were still in the apartment.
It is a tentative possibility that normally Madeleine would have been wearing shoes, which according to very old research would have prevented a trail being laid down.
Finally, it takes about 5 steps for a dog to determine direction.
The above does not prove anything, and is given for information purposes only.