Ok, don't tease me for knowing the answer to this. A friend of mine is friends with the owner of a large SAR-Dog facility. If I remember correctly:
A scent dog is given a scent article of the *specific* person who is the subject of the search. These dogs are the ones on long leashes and are either called tracking or trailing dogs.
The cadaver dog is not leashed and is trained to scent for decomposing flesh. I believe that cadaver dogs do *not* receive a specific scent article.
I remember her telling me that there are also different "levels" of tracking dogs, kind of like specialties, I guess. The dogs are tested through all their levels. For example, you can have a dog that started as a scent dog later specialize in cadaver scent after it has passed the training and testing process.
So maybe in this case, Merlin followed Laci's specific scent while Trimble on the same path followed a cadaver scent.
Okay, I won't laugh at you, if you won't laugh at me. For purposes of the most efficient tracking scent article given to a "tracking" dog, it would seem that, if a shoe is available, a shoe would be that specific article. That leads me to believe that Merlin supposedly put his nose to the ground, and tracked by "shoe scent". Laci's "tracks" went in an opposite direction from that of other police dogs, and WALKED away from the house according to Merlin. If she walked, according to Scott, it was Christmas Eve. According to MPD, they think Laci was already dead. I'm sure I must not be making sense, but I make sense to me. ;^)
Makes sense to me!