As the Supreme Court gets ready to hear oral arguments in a case Tuesday that could determine if authorities can track U.S. citizens with GPS vehicle trackers without a warrant, a young man in California has come forward to Wired to reveal that he found not one but two different devices on his vehicle recently.

The 25-year-old resident of San Jose, California, says he found the first one about three weeks ago on his Volvo SUV while visiting his mother in Modesto, about 80 miles northeast of San Jose. After contacting Wired and allowing a photographer to snap pictures of the device, it was swapped out and replaced with a second tracking device. A witness also reported seeing a strange man looking beneath the vehicle of the young man's girlfriend while her car was parked at work, suggesting that a tracking device may have been retrieved from her car.

Then things got really weird when police showed up during a Wired interview with the man.

The young man, who asked to be identified only as Greg, is one among an increasing number of U.S. citizens who are finding themselves tracked with the high-tech devices.

The Justice Department has said that law enforcement agents employ GPS as a crime-fighting tool with “great frequency,” and GPS retailers have told Wired that they've sold thousands of the devices to the feds.

But little is known about how or how often law enforcement agents use them. And without a clear ruling requiring agents to obtain a "probable cause" warrant to use the devices, it leaves citizens who may have only a distant connection to a crime or no connection at all vulnerable to the whimsy of agents who are fishing for a case.

The invasive technology, for example, allows police, the FBI, the Drug Enforcement Administration and other agencies to engage in covert round-the-clock surveillance over an extended period of time, collecting vast amounts of information about anyone who drives the vehicle that is being tracked.

"A person who knows all of another's travels can deduce whether he is a weekly church goer, a heavy drinker, a regular at the gym, an unfaithful husband, an outpatient receiving medical treatment, an associate of particular individuals or political groups — and not just one such fact about a person, but all such facts," wrote U.S. Appeals Court Judge Douglas Ginsburg in a recent ruling that the Supreme Court will be examining this week to determine if warrants should be required for use with trackers.

Greg says he discovered the first tracker on his vehicle after noticing what looked like a cell phone antenna inside a hole on his back bumper where a cable is stored for towing a trailer. The device, the size of a mobile phone, was not attached to a battery pack, suggesting the battery was embedded in its casing.

The first GPS tracker found was slipped into a fabric sleeve, containing magnets, and placed on the underside of the vehicle in the wheel well of the spare tire. Photo: Jon Snyder/Wired.com

A week later when he was back in San Jose, he checked the device, and it appeared to have been repositioned slightly on the vehicle to make it less visible. It was placed on the underside of the car in the wheel well that holds a spare tire.

Greg, a Hispanic American who lives in San Jose at the home of his girlfriend’s parents, contacted Wired after reading a story published last year about an Arab-American citizen named Yasir Afifi who found a tracking device on his car. Greg wanted to know what he should do with the device.

Afifi believed he was being tracked by authorities for six months before a mechanic discovered the device on his car when he took it into a garage for an oil change. He apparently came under surveillance after the FBI received a vague tip from someone who said Afifi might be a threat to national security. Afifi has filed a suit against the government, asserting that authorities violated his civil liberties by placing the device on his vehicle without a warrant and without suspicion of a crime. His attorney, Zahra Billoo, told Wired this week that she's requested a stay in her client's case, pending a ruling by the Supreme Court in the GPS tracking case now before it.

Greg's surveillance appears to involve different circumstances. It most likely involves a criminal drug investigation centered around his cousin, a Mexican citizen who fled across the border to that country a year ago and may have been involved in the drug trade as a dealer.

“He took off. I think he was fleeing. I think he committed a crime,” Greg told Wired.com, asserting that he himself is not involved in drugs.

Greg says he bought the SUV from his cousin in June, paying cash for it to a family member. He examined the car at the time and found no tracking device on it. A month later, he drove his cousin’s wife to Tijuana. Greg says he remained in Mexico a couple of days before returning to the U.S.

The first GPS tracker, out of its sleeve. Photo courtesy of Greg.

It’s possible the surveillance began shortly after his return, but Greg discovered the device only about three weeks ago during his visit to Modesto. The device was slipped into a sleeve that contained small magnets to affix it to the car.

On Tuesday, Nov. 1, Wired photographer Jon Snyder went to San Jose to photograph the device. The next day, two males and one female appeared suddenly at the business where Greg's girlfriend works, driving a Crown Victoria with tinted windows. A witness reported to Greg that one of the men jumped out of the car, bent under the front of the girlfriend’s car for a few seconds, then jumped back into the Crown Victoria and drove off. Wired was unable to confirm the story.

The following day, Greg noticed that the GPS tracker on his own car had been replaced with a different tracker, this one encased in a clam shell cover attached to a large round magnet to hold the device to the car. The device was attached to a 3.6 VDC Lithium Polymer rechargeable battery.

There was no writing on the tracker to identify its maker, but a label on the battery indicated that it's sold by a small firm in Farmingdale, New York, called Revanche. A notice on a government web site last June indicates that it was seeking 500 of the batteries and 250 battery chargers for the Drug Enforcement Administration. A separate notice on the same site in 2008 refers to a contract for what appears to be a similar Revanche battery. The notice indicates the batteries work with GPS devices made by Nextel and Sendum.

A spokeswoman with the DEA's office in San Francisco, however, declined to say if the device on Greg's vehicle was theirs.

"We cannot comment on our means or methods that we use, so I cannot provide you with any additional information," said DEA spokeswoman Casey McEnry.

Second GPS tracker with clam shell casing and Lithium Polymer battery.

The second device on Greg's vehicle appears to be a Sendum PT200 GPS tracker with the factory battery swapped out and replaced with the Revanche battery. The Sendum GPS tracker is marketed to private investigators, law enforcement and transportation security managers and sells for about $430 without the battery. With the factory battery “it will last 7-15 days reporting every hour in a good cellular coverage zone,” according to marketing literature describing it, and it uses CDMA cellular communications and gpsOne location services to determine its location.

When this reporter drove down to meet Greg and photograph the second tracker with photographer Snyder, three police cars appeared at the location that had been pre-arranged with Greg, at various points driving directly behind me without making any verbal contact before leaving.

After moving the photo shoot to a Rotten Robbie gas station a mile away from the first location, another police car showed up. In this case, the officer entered the station smiling at me and turned his car around to face the direction of Greg's car, a couple hundred yards away. He remained there while the device was photographed. A passenger in the police car, dressed in civilian clothes, stepped out of the vehicle to fill a gas container, then the two left shortly before the photo shoot was completed.

The Obama administration will be defending the warrantless use of such trackers in front of the Supreme Court on Tuesday morning. The administration, which is attempting to overturn a lower court ruling that threw out a drug dealer's conviction over the warrantless use of a tracker, argues that citizens have no expectation of privacy when it comes to their movements in public so officers don't need to get a warrant to use such devices.

It's unclear if authorities obtained a warrant to track Greg's vehicle. While Greg says he's committed no crimes and has nothing to hide, the not-so-stealthy police maneuver at his girlfriend's place of employment makes it look to others like she's involved in something nefarious, he says. That concerns him.

It concerns attorney Billoo as well.

"For a lot of us, it's like, Well I'm not selling cocaine, so let them put a tracking device on the car of [a suspect] who is selling cocaine," Billoo says. "And I'm not a terrorist, so let them put the device on someone [suspected of being] a terrorist. But it shouldn't be unchecked authority on the part of police officers. If law enforcement doesn't care to have their authority checked, then we're in a lot of trouble."