U.S. forces may continue to track Afghans for years after the conflict is officially done. Palm-sized sensors, developed for the American military, will remain littered across the Afghan countryside — detecting anyone who moves nearby and reporting their locations back to a remote headquarters. Some of these surveillance tools could be buried in the ground, all-but-unnoticeable by passersby. Others might be disguised as rocks, with wafer-sized, solar-rechargeable batteries that could enable the sensors’ operation for perhaps as long as two decades, if their makers are to be believed.