The court ruled unanimously that Mr. Jones’s Fourth Amendment rights had been violated, but was divided on why that was so. In a concurring opinion joined by four justices, it was the long-term surveillance of Mr. Jones, made possible and even easy by technological advancements, that was the real issue.

But Justice Antonin Scalia, writing for the five-justice majority, held that the heart of the matter was the physical placement of a device on Mr. Jones’s “private property, for the purpose of obtaining information.” It was this “government trespass,” he wrote, that made the use of the tracker without a warrant an unconstitutional search.

Some saw problems with this.

“The trespass theory went too far, and not far enough,” said Peter Swire, a privacy-law expert who teaches at the Georgia Institute of Technology.

On the one hand, he said, “it didn’t go far enough in Jones, because it seemed to allow for tracking locations as long as no one touched the car.” But on the other, he continued, it went too far “because even tiny touches become illegal.”

For the lawyers in Saginaw, the ruling was the whole ballgame.

“But for Jones,” Mr. Gronda said, “this case would have not been filed.”

While the two lawyers were researching the issue — learning among other things that Saginaw’s revenue from parking tickets could add up to $200,000 a year, one $15 or $20 ticket at a time — Ms. Taylor came across a Facebook post by Mr. Ellison. She said he was “kind of venting about the ticketing process,” and she joined in the comments beneath the post with grievances of her own.

“One, nobody should touch my car,” she said, listing her objections to chalking. “Two, it’s not a reliable source to get information. Because it’s chalk, you know.”