Governments’ use of purchased location data has exploded in recent months, as officials around the world have sought insights on how people are moving around during the Covid-19 pandemic. In general, governments have assured their citizens that any location data collected by the marketing industry and used by public health entities is anonymous. But the movements of a phone give strong clues to its ownership—for example, where the phone is located during the evenings and overnight is likely where the phone owner lives. The identity of the phone’s owner can further be corroborated if their workplace, place of worship, therapist’s office or other information about their real-world activities are known to investigators.