Deborah Levi, a public defender in Baltimore, knew something didn't sound right.
Her client Kerron Andrews, who was charged with attempted murder, told her that he kept hearing funny sounds on his cellphone before police arrived at the residence of an acquaintance and arrested him. But there was nothing in the police report, Levi said, about police using Andrews' cellphone to track him to that address.
Suspicious, Levi filed a series of motions with the court aimed at determining if police had, without a warrant, used his cellphone as a "real-time tracking device" to get a fix on his location.
"We couldn't explain it," Levi told NBC News. "If (the cellphone) wasn't used then how did you find him?"
The motions ultimately produced evidence that police had indeed tracked Andrews' cellphone through technology that mimics a cell tower to pinpoint a location. That led to a major Maryland Court of Special Appeals ruling last week that such tracking of suspects without first obtaining a warrant is unconstitutional.
The decision could potentially force a re-examination of hundreds of convictions in Baltimore and serve as an example for law enforcement nationally.
Because the ruling delves into thorny Fourth Amendment issues of unreasonable search and reasonable expectation of privacy, the issue could eventually end up at the U.S. Supreme Court, some legal experts say.
"As a matter of policy, a rule that a warrant is usually required for the government to use a cell-site simulator seems sound," Orin Kerr, a law professor at George Washington University of Law School wrote in a blog post for the Washington Post this week. However, Kerr finds the court's reasoning that government use of such devices are always subject to Fourth Amendment search protections—and thereby necessitate a warrant—"frustrating".
"Imagine the cell-site simulator reveals that Andrews is walking down a public street nearby talking openly on his phone. Or imagine he was found inside a vacant home that he had illegally entered. Why would the Fourth Amendment be implicated in those cases?" Kerr writes in his blog piece.
So-called "cell-site simulators" were originally developed for use by the military and intelligence agencies, said Nathan Freed Wessler, a staff attorney with the American Civil Liberties Union's Speech, Privacy and Technology Project.
In more recent years, the tools have trickled down to federal, state and local law enforcement.
The devices, commonly called Stingrays, effectively work like a high-tech version of the children's game "Marco Polo," Wessler said. By pretending to be a cell tower, the simulators effectively yell out "Marco!" to all the cell phones in an area on a given network. Phones on that network respond with their identity and location, Wessler said.
"These devices also gather information from large numbers of potentially innocent bystander phones," Wessler said.
The ACLU has seen cases where Stingrays were used to pinpoint a phone's location with surprising accuracy, he said, including in a parked car on a busy road, a bus driving down a street, and to an apartment in a large complex. Law enforcement will sometimes use a powerful version of the technology mounted in a police car to close in on the device, then use a handheld version to further narrow their search.
According to an ACLU report, at least 61 agencies and Washington D.C. have Stingray devices. However, non-disclosure agreements between the Harris Corporation, which produces the Stingray technology, and law enforcement, makes tracking usage tricky.
It's a useful tool for police - which means it should be subjected to oversight, not discarded entirely, Wessler said.
"It's a useful and powerful tool, so the question isn't so much about closing the door totally on it," Wessler said. "It's about bringing it out into the public for scrutiny by the courts, and by legislators, and by lawyers, and subjecting it not only to warrants but also to some other requirements."
Those requirements could include rules around how long data can be held, the deletion of information scooped up on bystanders' phones and requiring regular reports on how frequently Stingrays are used.
In the Maryland case, the court was also concerned by an implication that carrying cellphones implies consent to being tracked and noted that it joins "in the view shared by other courts that, 'the fiction that the vast majority of the American population consents to warrantless government access to the records of a significant share of their movements by 'choosing' to carry a cell phone must be rejected'."
"It's virtually impossible today to participate in commercial and social and family life without a cellphone," Wessler said. "To suggest that people have some meaningful choice between carrying a cell phone and not, it's just such a facetious argument from the government."
The Maryland case could be potentially appealed to the state's high court over the next few weeks. The state is determining how to proceed.
"Each case is being individually reviewed and based on the circumstances of the particular case, will determine how we proceed," Rochelle Ritchie, director of communications for the Baltimore City State's Attorney's Office told NBC News.
In the meantime, Levi hopes that the Maryland ruling will serve as a cautionary tale for law enforcement.
"Other jurisdictions will look to this decision as guidance," she said. "We hope that police departments become more cautious. We have to trust that now there's a written decisions, state departments and police departments across the country will be more circumspect."