WASHINGTON — A sophisticated airborne radar system developed to track Taliban fighters planting roadside bombs in Afghanistan has found a new use along the U.S. border with Mexico, where it has revealed gaps in security. Operated from a Predator surveillance drone, the radar system has collected evidence that Border Patrol agents apprehended fewer than half of the foreign migrants and smugglers who had illegally crossed into a 150-square-mile stretch of southern Arizona. The number of "gotaways," as the Border Patrol calls those who escape apprehension, is both more precise and higher than official estimates. According to internal reports, Border Patrol...