In an enormous grocery store in northern France, the lights above the aisles aren't all they seem to be. They look ordinary--more than a mile and a half of fixtures exuding bright light, folded into a grid overhead--but they're actually flickering faster than the human eye can see. The unique patterns each individual section of lighting emits are a 21st-century twist on Morse code, meant not for people, but for the cameras on their phones. If shoppers grant the store's app access to their smartphone's front-facing lens, the phone can watch for the lights and use the pulses to pinpoint its location. Doing so allows the app to plot the best routes for shopping lists, tracking people as they travel through the store.