I Spent the Night With Yelp's Robot Security Guard, Cobalt

The newest member of Yelp's security team awakes just after 8 pm, ready to begin its rounds. It traverses the lobby, gliding over polished concrete toward a small recess in the corner, where it inspects the emergency exit tucked inside. Last year, burglars tried to breach the office by rending the door from the building, frame and all. The low-resolution camera mounted in the lobby saw nothing. "We couldn't see what was going on inside the alcove," says Rick Lee, Yelp's head of security, who joined me on my late-night visit to one of the company's San Francisco offices.