A couple of years ago, the city of Baltimore launched a pilot program (no pun intended) that involved having a private contractor fly a surveillance plane over the city almost constantly, recording high-resolution video of the activity on the streets. It was surprisingly successful, resulting in police being able to retrace the movements of people engaged in shootings and other crimes back to their homes or to their post-crime destinations.The public wasn’t initially informed of the program and when word leaked out there were complaints from the usual digital privacy advocates. The program was eventually terminated. This year, however, the...