Different dogs are trained to scent on different things. Some dogs track live persons, so they are trained to follow certain living scents. Others are trained specifically to locate dead people (some even trained to locate bodies in water) where the biological gases and chemicals given off as a body decomposes, overwhelm or mix with the scent of a living person. The cadaver dogs are more specifically trained to pick up the chemicals of a decomposing body, as opposed to the unique scent of a living person.
If a cadaver dog alerts, it has found the presence of a dead body. It doesn't specifically denote which person, unlike the dogs that search for living people, which will only alert for the scent they've been offered via personal items of the missing person.
I have been looking and looking at information on cadaver dogs and I can't find the speficif reference that it has to be at least two hours for a body to be dead before a dog will alert. So I haven't been able to confirm if it is a scientific issue, that the body just does not issue enough chemicals until about two hours have passed (knowing that body immediately begins to decompose once life ceases) for a dog to detect, OR if it is more a case of the average dog does not alert reliably enough times until the body has, on average, been deceased at least two hours.
That would be a significant fact either way because it would change a possible time line. If a really good dog can alert sooner, it would narrow the amount of time a body would have be deceased. If it is more a chemical/mathemical kind of situation for the dog to detect, then the two hour timeframe is more certain.
I'm going to keep looking.