I've just read that a missing woman was found with use of a "scent preservation kit".
A sterile gauze pad is used to swipe the armpits. The pad is placed in a special jar, apparently sealed with tamper-proof tape and labeled with instructions.
This kind of tracking technique was used to locate a missing woman in Florida. The scent had been preserved in a jar for 2.5 years.
A short 7-27 article in USA Today described how a bloodhound in 5 minutes located the woman, who had been missing about 2 hours.
No indication was given on how far away the woman was when she was located. I guess that depends on the ability of the tracking dog.
Perhaps this will become a standard wander management technique for locating people who wander.