RECENTLY, IT HAS BEEN GETTING HARDER to disappear on this planet. A surveilling drone began passing over the remote forests of northeastern Nigeria earlier this year, tracking the separatist group Boko Haram, catching glimpses of hasty encampments and escapes along dirt trails. When the militants kidnapped 200 schoolgirls this spring, a camera in the sky captured a large group of girls, sitting together in a clearing. Soon after, the cameras captured a similar group of girls elsewhere in the forest. Each time, the girls were moved before they could be spotted again, or rescued. What was left was just a spooky afterimage, like the impression made on a photographic plate: The most famous missing people on the planet, for an instant at least, found.