If there is something Border Patrol agents know, it is a process for tracking border crossers called “sign cutting”. One definition of sign (not mine) is “physical evidence of disturbance by the passing of people, animals or objects.” Sign is one of the most important resources Border Patrol agents utilize to figure out that desert-crossing immigrants or drug smugglers have passed by. And agents had better be good at it. It is their jobs to track and interdict individuals who have crossed into the United States.
Consider the following example from Billy Maddox Takes His Shot.