Whenever you walk around a major Western city with your phone’s Wi-Fi turned on, you are broadcasting your location to government agencies, marketing companies and location analytics firms. In shopping malls, for instance, a firm called Euclid Analytics collects, in its own words, “the presence of the device, its signal strength, its manufacturer (Apple, Samsung, etc.), and a unique identifier known as its Media Access Control (MAC) address.” In London last year, one start-up installed a dozen recycling bins that sniffed MAC addresses from passers-by, effectively tracking people through the area via their phones. Such companies go to great lengths to explain that such information in not personally identifiable—except that repeated studies have shown that this data can indeed be used to infer a great deal about your life.