"The dog can detect things you can't see, even if it's a crime scene where someone had cleaned up blood. If there's a drop of blood that's been wiped up and you can't see it, the dog will find that. Then you bring in your forensic staff and let them concentrate on that spot so they can quickly determine if there's blood evidence in the house."
Paul Dotsie - Buster's handler
K9 CRIME-FIGHTER: BUSTER THE 3-LEGGED DOG FINDS BURIED BODIES
https://crimewatchdaily.com/videos/0-dmok8ljg/
Start from the premise that all dogs detect all scents, all the time.
What a dog reacts to is determined by training.
All dogs detect the scent of drugs.
But only dogs trained to react to the scent of drugs will do so.
All dogs detect the scent of cancer-cells in humans.
But only dogs trained to react to the scent of cancer-cells in humans will do so.
All dogs detect the scent of money. But only dogs trained to react to that scent will do so.
And so on.
Eddie would react to the scent of blood, because he was introduced to blood as a discrete scent and rewarded for doing so.
When Keela, the younger of the two dogs Eddie-and-Keela, came along, she was trained to react, only, to human blood.
It was always the weak link in the Eddie-and-Keela partnership that both dogs react(ed) to the scent of blood.
When Grime trained Morse, he skipped the step of introducing Morse to the scent of blood as a discrete scent, so that the dog would only react to blood if it was mixed with a human corpse which, alone, is what he was trained to react to.
The idea was that by not training the dog to react to blood as a discrete scent you would increase assurity that if the dog did react, the reaction was to a cadaver scent.
At the same time, in the event of a reaction by the cadaver dog, you had a second dog trained to find nothing but blood who could search an area made manageable to search by the reaction of the cadaver dog looking for blood (that may be of assistance in a criminal enquiry).