You can find just about anything at the annual homeland security expo: X-ray machines, infrared cameras, a police cruiser with heat-sensing capability, a hovering "gyroplane" -- and a GPS device that can spy on your spouse.

The salesman for Blackline GPS Corp., maker of "professional-grade covert tracking" equipment, explained that his devices, in the shape of a legal envelope ($700) or an electric razor ($300), can be tucked behind seat cushions, under floor mats or into backpacks.

"We're getting more requests from husbands and wives," he explained. "I've seen guys throw it in their wives' car and cover it with a hat. It keeps honest people honest."

That, in one convenient package, is what has become of the homeland security effort. What began as a well-intentioned campaign to harden targets and protect the nation from terrorists has metastasized into a sprawling and diffuse enterprise that has little to do with terrorists and a lot to do with government and employers spying on the citizenry -- and citizens spying on one another.

The GovSec expo this week at Washington's convention center reflects the shift. Billed as "the premier government security event," it began after the 9/11 attacks, its organizers told me, with vendors hawking security barriers, razor wire and the like. Now the 2,500 conventioneers can visit the booth of a vendor called ECM Universe, which specializes in monitoring Twitter.

Its "social media surveillance" package helps universities monitor online activity for evidence of bullying, among other things, ECM's Scott Raimist told me Tuesday. Two weeks ago, the company helped authorities in Fort Lupton, Colo., identify a man who was tweeting such menacing things as "kill people" and "burn [expletive] school." Said Raimist: "He fit the profile of a pyromaniac."

So is the man behind bars? Well, no, Raimist admitted. "He's outside their jurisdiction. He's still tweeting." In fact, the man hasn't been accused of a crime -- but that didn't matter: His full name was projected on a display screen at the GovSec expo as an example of how technology can catch bad guys.

Federal homeland security spending tripled in the years after Sept. 11, and recent cuts have been modest compared with reductions to other parts of the budget. States and private industry, too, have spent billions. But that money is going further and further afield.

Government agencies and corporations are, for example, buying "Pocket Hound" cellphone detectors, which indicate who is carrying a mobile phone (among the suggested uses: schools and airports). A competitor, Cellbusters, can locate where a cellphone is inside a building or whether someone in your conference room is violating a company's no-cellphone policy.

Catch many terrorists with this technology? "Not so much," Cellbusters' Derek Forde admitted.

Neither is Fulcrum Biometrics likely to apprehend Al-Qaida operatives with its ID system using fingerprint, face, iris, palm and voice identification. Recommended uses include voter registration and "civil ID," said Fulcrum's Kathleen Erickson.

Also, gym memberships: "You can use it in guest management, like a loyalty program." One product enables employers to require construction workers and others at remote sites to clock in with their fingerprints.

"Can I scan you?" Erickson asked me. She waved a scanner at my convention badge, and with a "boing" sound my registration information was transferred to her.

There are, of course, legitimate uses for all such gizmos, as there are for gun vaults, portable bunkers and military gear. But Big Brother's display space at the expo is expanding.

Emergency Vehicles Inc. can convert a Honda Odyssey minivan into a "covert surveillance platform" with heat-detecting cameras. "They can focus in on a person and follow that person wherever they go," explained salesman Michael Cox.

A company called Telmate sells a kiosk that records and photographs prison inmates during conversation, games or religious services. Hunt Engineering enables agencies and businesses to scan driver's licenses or passports and run background checks before admitting visitors. Gamber-Johnson is offering a GPS-enabled laptop dock that allows a company to map an employee's travel for a month.

Nearby, International Surveillance Technology is selling hidden cameras and audio recorders in alarm clocks, iPod docks, water coolers and suitcases. Among government security agencies, "there's nobody who isn't buying this," said chief executive Donald DiFrisco. "Imagine: hookers in a hotel room with a clock radio."

That's the homeland security mission creep: from Osama bin Laden to hookers in hotels.