Contact tracing, recording close proximity between people using bluetooth, wifi, or gps data, could help efficiently notify people that they have earlier been in contact with someone now diagnosed with coronavirus and should self-isolate. The typical design for such an app would either be to have the phone detect nearby phones, e.g. through Bluetooth like in Copenhagen and Boston, or through GPS, with phones sending the data to a centralised server which would then warn users when a case is confirmed. While potentially very effective, this also enables the collection of an extremely large amount of sensitive data. In a thought experiment two years ago, we estimated that 1% of London installing a malicious app could allow a digital attacker to track half of the London population1 potentially without their consent.
Anonymizing location data is always very hard.