Dallas Police Sergeant James Johnson first started playing around with bait cars when he was working on auto theft cases for the DPD. Over the past decade, the department rigged vehicles with GPS trackers and set them up to be stolen. His team put out eight cars (at most) across the city, selecting small hotspots at high risk for auto theft.

Johnson would watch the map, wait for the car to start moving and follow it to a home or a chop shop, where his squad would roll in and make their arrests. Each year, they'd nab about 50 to 60 people; the program is credited with cutting auto theft by 57% since 2014.

But today, it's not just cars that are used as bait. Using the latest GPS trackers, police have transformed a range of coveted objects — purses, bikes, boxes of Air Jordans, UPS packages sitting on a stoop, bottles of prescription drugs, anything someone might want to steal — into trackable objects.

In 2014, Johnson got his first contract as part of the Operational Technology Unit to begin building bait devices. His first year, there were 195 arrests. This year, his goal was 250. He blew way past it.

As Johnson spoke with Mic, he ruffled through his notes and updated the numbers from just the past 24 hours. On Monday afternoon, a coil of copper wire that was rigged as bait led officers to a scrap yard, where a thief was attempting to resell it. After adding in those arrests, the total came out to 371 so far in 2015.

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Police surveillance used to require grit, sweat and time. Traditional stakeouts could last entire shifts, with multiple officers waiting through the night with cups of coffee, watching an expensive bait car and hoping it got stolen. Even when police started incorporating tracking technology like beepers, police surveillance was individualized. Choosing to surveil someone took up so much time and resources, police had to be sure they were targeting the right people.

"This has always been practical, built-in restriction, because if you're assigning an officer to do that, they're not doing something else," Rachel Levinson-Waldman, senior counsel for the Brennan Center for Justice, told Mic. "It was a structural limitation on how long that surveillance was going to go on and on how many people can be subject to it."

But over the past 10 years, GPS devices have flourished and become cheaper than ever, allowing a department to set dozens of traps and lie in wait.

Jason Cecchettini, president and CEO of Pegasus Technologies, says that when he started the company, in 2002, it could cost upward of $8,000 to set up a single bait device, a prohibitive cost for smaller departments who wanted to run GPS-based stings. Now, for a sophisticated, nimble tracking set-up, the entry-level price is $1,400. For the cost of a full-time officer or detective with a median salary of about $53,000, you can build an entire swarm of devices that ping their owners only when an "activation" occurs (that is, when something is stolen).

"Our first radio transmitter back in 2008 was probably the size of a credit card and an inch thick," Cecchettini told Mic. "Now we have one the size of your pinky nail."

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The best bait: Pegasus sells tracking devices, which police officer can install in whatever they want: luxury goods that might lead officers to a fencing operation or industrial items like portable generators. But law enforcement's favorite object for reeling in thieves are bikes, as cycles are a glittering lure for more than just bike thieves. Police can place a bike in a truck bed where there are high rates of auto theft or alongside a garage while burglars case a neighborhood for home break-ins, and often, the first thing these criminals will snatch is the bicycle.

Because of the reduced prices, Cecchettini's biggest customer base has become local college police departments and small towns. University of Wisconsin-Madison cut their bike thefts down by 40% in their first year, and at the University of California at Berkeley, they reduced on-campus thefts by 45% and arrested 31 thieves in the first half of 2015.

A bait bike typically has two pieces, placed in separate locations in case a criminal finds one of them. The first is a GPS unit, with which police can immediately find out if something is on the move and locate it to within 4-10 meters. The second tracker uses older, clunkier radio-frequency tracking that allows police to pinpoint a more precise location directionally, similar to how a metal detector sweeps a beach.

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Trick or trap: In the Harvard Law Review's "Bait, Mask, and Ruse," Elizabeth Joh of the U.C. Davis School of Law argues that allowing police to hide themselves behind various digital veils — whether it's by using a bait device or posing under a fake identity on Facebook — creates opportunities for police to creatively trick people into committing crimes they otherwise wouldn't.

"The majority of courts don't permit you to raise a successful entrapment defense if you're predisposed to commit that offense anyway, so entrapment law has never been a winning defense for most criminal defendants," Joh told Mic.

In cities like San Francisco, where the bike theft unit, whose logo looks like the insignia of a bike gang, with a skeleton on a bicycle, is boastful about their success — bragging about their arrests on Twitter and defending themselves on Twitter — journalists and activists argue that the policy of using bait devices is seen as a way of targeting the poor and the economically vulnerable.

Activists argue that the policy of using bait devices is seen as a way of targeting the poor and the economically vulnerable.

In most cases, even staunch warriors for privacy and civil liberties will admit that police are mostly doing well when it comes to staying on the right side of ethics and the constitution when it comes to bait devices.

"The basic criteria should be if police are trying to catch crime happening on its own, or are they trying to create crime where it otherwise wouldn't happen," ACLU senior policy analyst Jay Stanley told Mic. "I don't think a bait object — if used in the way everyone imagines it's used, which is to catch a thief — is a civil liberties problem."

But Stanley does admit one potential caveat. For most operations, police wait for a criminal to take the bait, and head right out to arrest the thief. But occasionally, detectives (or sometimes even television journalists on programs like Inside Edition and Dateline) need more than a quick hit, and will ask companies like Pegasus Technologies for the tools to undertake long-term investigations.

This is where police surveillance begins to step past a high-tech stakeout and into the realm of sustained surveillance programs that are advancing in technical sophistication faster than the law can stop them.

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What is a "search," anyway? The barriers to constantly surveilling someone used to be far higher, simply because they required so many dedicated resources and working hours that police only followed someone for long enough if they had a good reason. So as devices became slimmer and cheaper through the 2000's, there were years without any legal precedents or processes keeping officers from simply leaving one of those trackers with someone and collecting information on their whereabouts for weeks at a time.

And then someone took it too far. During a 2004 narcotics investigation of Washington, D.C., nightclub owner Antoine Jones, Metro Police teamed up with the FBI and eventually placed a GPS tracking device under his car, monitoring his location 24/7, for a period of four weeks, without a warrant. The Supreme Court ruled that sustained surveillance without a warrant was a violation of the Fourth Amendment.

It was hailed as a victory for privacy advocates. Police now need a warrant for leaving a GPS tracker with an alleged criminal for extended periods of time, and the ruling instantly put a hold on what some lawyers described to Mic as "thousands" of FBI and police investigations. But while it addresses that very narrow instance of putting a tracker on someone's car, the Jones decision ends up being a monkey wrench in the history of Fourth Amendment law: It leaves us unprepared for the future of connected devices.

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A brief history of invading your space: For most of American history, the Fourth Amendment, which prevents warrantless search or seizure, was all about tactile boundaries like the walls of a house or the confines of your property. Police executing a search, going into your car — this very physical interpretation of the Fourth Amendment is called "trespass theory," and the litmus test for a Fourth Amendment violation was based on whether law enforcement officials intruded into your personal space.

Then, one day in 1967, Charles Katz, one of the nation's most prolific gamblers in basketball, strolled out of his apartment on Sunset Boulevard in Los Angeles and down to a public payphone to call his bookie. The FBI had disabled one of these phone booths, forcing him to use a booth that had an electronic listening device fixed to the outside.

The agents had no warrant to tap his communications. After all, they weren't entering his private space, so why would they need one? But when Katz's attorneys took his case before the Supreme Court, they won. They successfully argued that the "average, reasonable man" would assume his payphone communications were private. Technological surveillance transformed the face of the law, and "reasonable expectation of privacy," a new standard for protections under the Fourth Amendment, was born.

"It used to be that if you had a spike mic that went physically through the wall into the next room, it was a search, but if you didn't use a spike it wouldn't be — it was physically based," Fourth Amendment scholar Andrew Guthrie Ferguson told Mic. "In the Katz case, they changed the doctrine. We're worrying people, not places. Up until Jones, that was the controlling theory."

"In some of these high-tech cases ... surveillance is done without any physical intrusion at all." — Andrew Guthrie Ferguson

But those two supreme court cases in which technology went too far in using surveillance — the wiretapping of Katz in 1967, and the GPS tracking of Jones in 2012 — used totally separate philosophical doctrines to roll back police search. When Justice Antonin Scalia wrote the majority opinion, he wasn't really protesting the invasion of Antoine Jones' privacy. Instead, Scalia brought back trespass theory: His objection was that police had installed a GPS device on the physical car itself, not that it violated Jones' personal privacy.

Now, the rules around what police can and can't do with technology are stuck between two competing ideas: that the police shouldn't be able to warrantlessly track you by invading your physical space, or that they shouldn't be able to track you when you can reasonably expect privacy. Because the problem with police use of GPS devices isn't ultimately that they're buying them to place on us, but that we're all buying GPS devices that track us on our own — we're just used to calling them cellphones.

"It's hard to find firm lines, and the court has been criticized for creating a muddle of a doctrine," Ferguson told Mic. "There's a gaping hole in some of these high-tech cases where surveillance is done without any physical intrusion at all."

Which brings us back to the ultimate irony of the Jones case: The police never needed to place a tracker on Jones' car because they'd easily obtained a warrant for all his GPS information using the cell site data from his phone. They'd just haphazardly allowed it to expire, and if the FBI had simply let his phone provider do the surveillance for them, there'd be no Jones ruling at all.

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What this means for you: Despite all of those battles in the Supreme Court over when and how police use bait objects, there's an even easier tracking method: asking for information people have already given to their cellphone provider. In the Jones opinion, Justice Sotomayor had this warning about the next frontier of protecting our location information from police officers:

More fundamentally, it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties. This approach is ill suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks.

Police are taking advantage of the "third-party doctrine," a rule stating that you relinquish your privacy if, for example, you confessed your crimes to a close friend or co-worker and put that info in the hands of a third party. That person would be legally justified if they chose to take it to the police. Now, law enforcement is attempting to use that premise to argue that information gathered by your cellphone provider should constitute a third-party relinquishment, as if Verizon and AT&T were your willfully trusted confidants.

We've been building the rules around police use of GPS trackers — and many of the lawyers we spoke to agree, the future of privacy looks bright in that regard — the third party doctrine has been expanded, allowing law enforcement to go after our personal information wherever its stored — cell phone records, medical records in state databases, confessional activity on social media, wherever.

"Even if I turn off my GPS, my apps don't know where I am, but my phone company still knows," ACLU staff attorney Nathan Freed Wessler told Mic. "By just walking around, you're not necessarily consenting to a phone company knowing [your location] and telling the police."

And now, Internet-connected devices are everywhere. Onstar keeps track of your car's travel information, Internet-connected homes want to wire your door locks and lightbulbs to the web, and information gleaned from Fitbit data has been successfully entered into evidence as proof of a person's activity.

When we take on trackers, whether for convenience or necessity, we invite third parties to be privy to our information that neither reasonable expectation of privacy or the safety of four sturdy walls can protect — taking the bait without the police having to lay it out themselves.

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