Imagine that you share a ride to work with the same colleague most mornings. As it passes by a license plate reader, the camera records the car, which can be linked through vehicle records to its registered owner. Beside it, another sensor detects signals broadcast by devices traveling nearby, such as your phone and your colleague's smartwatch.

Imagine that you share a ride to work with the same colleague most mornings. As it passes by a license plate reader, the camera records the car, which can be linked through vehicle records to its registered owner. Beside it, another sensor detects signals broadcast by devices traveling nearby, such as your phone and your colleague’s smartwatch.