At least one of the original dogs had some training in tracking, so he possibly could sniff and use the scent. So possibly did scent something?
In general I think it was air scenting dogs that were used.
A wee bit of info
AIR SCENT DOGS
The air scent dog is the type most frequently encountered. This dog finds lost people by picking up traces of human scent that are drifting in the air, and looks for the "cone" of scent where it is most concentrated. This dog will not normally discriminate scents, so there is the possibility of a "false alarm" if other people (searchers, citizens) are nearby. Airscent dogs work best in situations such as large parks or private lands that are closed at the time, since the dog will home in on any human scent. The success of an air scent dog will be affected by a number of factors, including wind conditions, air temperature, time of day, terrain, and presence or absence of contamination (auto exhaust, smoke, etc.). The best conditions for air scent dogs to work are early mornings or late afternoons on cool, cloudy days when there is a light wind.
TRAILING DOGS
The trailing dog
is often referred to as a "tracking" dog, although "tracking" and "trailing" are not the same to the purist. The trailing dog is directed to find a specific person by following minute particles of human tissue or skin cells cast off by the person as he or she travels. These heavier-than-air particles, which contain this person's scent, will normally be close to the ground or on nearby foliage, so the trailing dog will frequently have its "nose the ground," unlike the air scent dog.
A Bloodhound is typically trained for scent discrimination. Each dog is usually worked in a harness, on a leash, and given an uncontaminated scent article (such as a piece of clothing) belonging to the missing person. The dog follows that scent and no other. At times, the dog may track, following the person's footsteps, or air scent, and home in on the subject's scent.
Field contamination (scent of others) should not affect his work. He should be able to trail scents on pavements, streets, grass, water, etc. If there is a good scent article and a point where the person was last seen, a trailing dog can be the fastest way to find the victim. Without the scent article and a point where the person was last seen, these dogs cannot work effectively.
While those are the two standard types of search and rescue dogs, there are also other dogs trained to find lost people.
TRACKING DOG
A tracking dog is trained to follow the path of a certain person. It physically tracks the path of the person, without relying on air scenting. This dog is usually worked in a harness and on leash. This type of dog is effective when pursuing an escaped criminal if no scent article is available. These dogs are also used successfully in search and rescue operations.
http://www.ussartf.org/dogs_search_rescue.htm
