If the dogs tracked the scent to the water's edge to blood or whatever else was washed off in the lake, wouldn't they also be tracking the perps scent?
From the point of contact with the girls to wherever the perp went (with or without the girls) would the dogs be tracking the girl's scent or the perps scent?
If girls went or were taken one direction, maybe to a vehicle, and perp went another direction, maybe to lake or woods or trail, would the dogs be able to distinguish the 2 scents?
No.
A trailing dog is the type of dog the FBI flew in. Such dogs are trained to find and follow only the person who was on the scent item. If there was more than one person who may have handled the scent item, then that person or persons are lined up and the dog is given the problem "follow the person whose scent is not here right now."
To a dog, the perp would be incidental. The dog doesn't care what the perp did, the dog cares about figuring out what the person whose scent they are looking for did. Or, more specifically, the dog cares about what that particular scent did.
There are dogs that are trained to indicate on the problem of "who does not belong here?" The way such dogs are trained, all the people who were known to be in a certain area are lined up and the dog indicates on any scent they find that does not belong to someone in the line up.
This is useful in a situation where LE isn't sure if an unknown person was in a house or other private area.
This is not useful for a public area like Meyer's Lake because there's just no way to line up the dozens or hundreds of people who used the trail innocently. The more people in a cattle call, the more likely it is one or more won't be there. It's just human nature.
What is frustrating is that undoubtedly the dogs know what it is all the scents say. Who was excited, who was bored, who was scared, etc. The problem is that we humans can't communicate well enough with dogs to get the answers to such questions.
Right now, the best we can do is train dogs to answer relative simple, basic questions, such as "has someone been in this room that is not here right now?" If there is some way to get an answer by asking a yes/no question, a dog can be trained to do it.
The shortcoming is human rather than canine.