The ability to secretly “tail” or follow a suspect is an important police and security skill. The purpose of tailing is to identify people the suspect talks to, people he makes exchanges with, places where he drops or hides things, and to where he goes.
The better trained and more aware the target, the more difficult he is to follow without being alerted. Enemy operatives are trained to go to extremes to lose suspected tails or make themselves difficult to follow. FBI agents followed John Walker for several hours while he leisurely wound his way from Norfolk, Virginia to the Washington, D.C. suburbs before making the secret drop that immediately led to his arrest.
Special units are trained to be tails, both on foot and in vehicles. Normally this is a team operation with many people and vehicles involved. A wary suspect constantly checks his rear looking for familiar faces and cars. When enough assets are available, teams are deployed in front and to the sides of the suspect so that he is constantly boxed in. Backup teams rotate so that the suspect never gets a repeated look at the same followers. When possible, agencies employ helicopters for aerial surveillance of vehicles.