The United States Customs and Border Protection (CBP) is now patrolling half of the US-Mexico border with drones, according to a new report by the Associated Press.

According to two anonymous sources from within the CBP, Predator B drones fly over remote areas with "a high-resolution video camera and return within three days for another video in the same spot." Then, those videos are compared to see if there was any difference—footprints, livestock tracks, or vehicle trails.

Agents have found that most of the time, nothing has changed. Only 2 percent of the drone missions did offer evidence of unauthorized border crossings, and the CBP usually places more detailed "ground sensors" in those areas. False alarms were reported for four percent of the missions, while two percent of missions were inconclusive.

This program, known as "change detection," first began in March 2013—at present it only covers the US-Mexico border, but it's set to expand to the Canadian border in 2015.

Lothar Eckardt, the agency's executive director of national air security operations, told the AP that "law-abiding people shouldn't worry and that cameras are unable to capture details like license plate numbers and faces on the ground."