For information:
“Not all SAR dogs perform the same type of search. Some dogs are tracking (or trailing) dogs, and others are air-scent (or area-search) dogs. The types overlap, but the distinction between the two guides are the training process and how the dog participates in missions. Tracking dogs work with their nose to the ground. They follow a trail of human scent -- typically heavy skin particles that fall quickly to the ground or onto bushes -- through any type of terrain. These dogs are not searching, they're following: Tracking dogs need a "last seen" starting point, an article with the person's scent on it to work from and an uncontaminated trail.
For tracking, time is an issue. If a child disappears from a school playground or a inmate escapes from a prison, a tracking dog might be called in to follow the person's scent immediately after the disappearance, before other search groups and law-enforcement personnel contaminate the scent trail.
Air-scent dogs, on the other hand, work with their nose in the air. They pick up human scent anywhere in the vicinity -- they don't need a "last seen" starting point, an article to work from or a scent trail, and time is not an issue. Whereas tracking dogs follow a particular scent trail, air-scent dogs pick up a scent carried in air currents and seek out its origin -- the point of greatest concentration.
Air-scent dogs might be called in to find a missing hiker located "somewhere in a national park," an avalanche victim beneath 15 feet of snow or people buried under a collapsed building”. (animals.howstuffworks.com).