The unsettling thing about living in a surveillance society isn't just that you're being watched. It's that you have no idea.

That's what struck me about a story told last week by a border agent at a meeting of 200 San Juan Islanders. He was there to explain why the federal government is doing citizenship checks on domestic ferry runs.But near the end, while trying to convince the skeptical audience that the point is to root out terrorists, not fish for wrongdoing among the citizenry, deputy chief Joe Giuliano let loose with a tale straight out of "Dr. Strangelove."

It turns out the feds have been monitoring Interstate 5 for nuclear "dirty bombs." They do it with radiation detectors so sensitive it led to the following incident.

"Vehicle goes by at 70 miles per hour," Giuliano told the crowd. "Agent is in the median, a good 80 feet away from the traffic. Signal went off and identified an isotope [in the passing car]."

The agent raced after the car, pulling it over not far from the monitoring spot (near the Bow-Edison exit, 18 miles south of Bellingham). The agent questioned the driver, then did a cursory search of the car, Giuliano said.

Did he find a nuke?

"Turned out to be a cat with cancer that had undergone a radiological treatment three days earlier," Giuliano said.

He added: "That's the type of technology we have that's going on in the background. You don't see it. If I hadn't told you about it, you'd never know it was there."