Every human constantly "sheds" odor into the environment, wherever they go.  Skin cells, respiration, sweat- all of these contribute to a particular person's scent.  Trailing dogs are taught to utilize this scent in order to follow the general path that a specific person has traveled through an area.  These dogs are generally worked on-lead and require a "clean" scent article from the subject so that they know which scent to follow.  Trailing K-9's are particularly useful in determining a direction of travel from the last place the subject was known to have been.  While there are exceptions, most trailing dogs are only reliable on trails up to about 72 hrs. old, depending upon conditions and the level of other scent contamination in the area.