Are they alerting to actual corpses or just old stale blood & ages old rotting body fluids tho?
Different dogs will have being trained to alert to different things, some will scent and alert to blood, but cadaver dogs scent and alert to decomposition of human remains, they can also alert to residual scent so alert if a body has been placed somewhere and then moved to a different location. they will not alert to animal remains or the like, only human. (And now I won't able to sleep tonight ) the dogs can also differentiate between living and deceased scents of people.
In the McCann case I believe one dog was trained to scent and alert to blood, the second dog to scent and alert to corpses.