Striding through San Francisco's busy financial district after dusk, 20-year-old Jake Appelbaum is an odd sight. His left hand is clutching the handle of a two-foot-long fiberglass pole wrapped in a metal spiral, which he holds high like a lance. The device is a directional antenna: a thin cable hangs between it and what looks like a handheld TV in Appelbaum's other hand.

As he walks, Appelbaum studies the fluttering static on the receiver's LCD screen while rapidly thumb-clicking a button below it, occasionally glancing up to avoid slamming into other pedestrians on the sidewalk -- most of whom stare as he passes. "You get the Playboy Channel on that?," one asks.

He doesn't. But at the corner of Mason and Post a clear black-and-white image flickers onto the 2.5 inch screen. It's the interior of an office: a clock and a piece of art can be seen above a desk cluttered with stacks of books. The view is angled sidewise and up towards a drop ceiling, and is partly obscured, giving the video feed a decidedly covert look. Watching the display, Appelbaum sweeps the antenna slowly, left to right, up and down, dowsing for the source of the signal, which seems to be emanating from an upper floor of a hotel. "That's a hidden camera right there," he says, with perhaps more confidence than is due.