If you live in Amsterdam, chances are you have come across a street sign that features the WhatsApp logo, followed by the word “buurtpreventie” (neighbourhood watch), which indicates that residents living nearby are using their smartphones to inform each other on perceived threats to their safety. The use of the popular messaging app for street-level surveillance by ordinary people is certainly not a local phenomenon, though. In cities around the world, WhatsApp groups for security have encouraged neighbours to keep their eyes on the streets, and on each other.