Do you know what type of dogs they were DS? From reading the Madeline McCann case it appears there are two types that are used. One for blood, etc. and one for the "death" smell. Doubt after all this time that a dog could get any type of scent unless maybe they let the dogs sniff the pants/sweatshirt or some other item of Brianna's??
littledeer,
cadaber dogs which are used to detect the smell of decomposed human remains, the scent from that decay will remain in the soil for several yrs, a well trained cadaber dog would be able to detect the scent, dogs are trained according to the main funtcion they will be used for, scent article dogs are commenly used to help in a search of a lost person for tracking purpouses. then they are trained for blood pathogene scent, and then cadaber dogs, for the smell of decay. the smell of decay might not last as long in areas that are prone to polluted water or runoff water from a roadway washing into an area where remains are, but in a remote spot as this is, the smell of decay would most likely be present, so depending on how long they spend out there increases the chance of finding something more or eliminating the area.