Search and Rescue Trailing K9s are trained to be scent-specific, meaning looking for 1 particular scent (person) at a time. They can help distinguish a direction of travel or if a subject was even present. Trailing K9s remain on a 20-30ft lead with a handler, working as a team.
When a person walks away, they are leaving their scent behind, which is a combination of sweat, oils, secretions by skin microbes, along with other things such as detergents and lotions. Scent can also be influenced by genetics, diet, and environmental factors.
*Fun fact: Humans shed 30,000-40,000 skin cells A MINUTE.
A trailing K9 will follow that particular scent. Trailing K9s do not follow footstep for footstep, but follow scent wherever it may be, which is influenced by weather, wind, temperature, moisture, and multiple other factors. These K9s can be deployed in an urban or wilderness environment.