The amended heels of the pedestrians’ wartime shoes pop metallically. Each pedestrian has a fairly consistent stride length and clicks with nearly metronomic precision. A microphone in the sidewalk would provide an eavesdropper with a cacophony of clicks, seemingly random like the noise from a Geiger counter. But the right kind of person could abstract signal from noise and count the pedestrians, provide a male/female break down and a leg-length histogram—