Robert Mueller says he was able to pinpoint security company executive Erik Prince’s precise location for several hours in January 2017 by matching his mobile phone signal to a cell site near Trump Tower in New York City. The special counsel’s report discloses the use of this investigative technique, by which police determine a suspect’s location via a cellphone’s GPS signal. The Prince narrative is one instance in unredacted sections of the report in which Mr. Mueller’s team explicitly discloses cellphone tracking. It raises the question of whether the FBI applied the process to other investigative subjects — a phone’s...