Your scathing restaurant reviews on Trip Adviser might not be as anonymous as you think. Just four geotagged posts are enough to identify an anonymous account from a phone company’s database of 10 million customers.

In order to effectively route calls to a phone, telecoms companies need to know which cell towers are nearby. In this way, they build up a crude map of each customer’s movements. A team led by Apostolos Pyrgelis at University College London examined these “fingerprints” to see if there …