For air scenting, it does not matter how many have been there before or how long since the subject has been missing. Many air scenting dogs are cross-trained to trail or track and can do so if the area is undisturbed and the track is not too old. Use bag turned inside out ot pick up item, or, use metal tongs or coat hanger. A search dog is useful for covering large areas fast, with the least amount of forces committed to the search. Some search dogs can work debris from floods, mudslides, tornadoes, explosions, plane crashes and earthquakes.
Some search dogs can work avalanches.
Some search dogs can search for evidence of crime. Search dogs can locate drowned people in the bottom of lakes, or in rivers, or all bodies of water. Gases escaping from bodies rise to the surface and a dog in a boat will give an accurate indication. These airborne cells and odors act like smoke. During calm days, with the sun over head, smoke and scent rise up from convective currents.