There was nothing particularly special about the Ford minivan that passed by a Kansas state trooper watching traffic on Interstate 70 one morning in May 2010, an appeals court said in recounting the traffic stop that would compel the court to confront a clash between technology and the Fourth Amendment.

The officer called in the license plate—a Colorado 30-day temporary registration tag. A dispatcher reported "no return"—indicating, prosecutors would later argue, that it might be forged or that something else was amiss. The vehicle stop ensued. The driver consented to a search. In a secret compartment near an air vent on the dashboard, the trooper found more than a pound of methamphetamine.

But there's the problem, the U.S. Court of Appeals for the 10th Circuit ruled on August 2, returning the case to the trial court for a fuller examination of the traffic stop. The dispatcher told the trooper that Colorado temporary tags "usually don't return." The database, the appeals court said, "might bear a real problem—a problem that might mean 'no return' doesn't suggest criminal conduct but only some bureaucratic snafu."

"Garbage in, garbage out. Everyone knows that much about computers: you give them bad data, they give you bad results," 10th Circuit Judge Neil Gorsuch wrote. He added: "But what if the computer turns out to be a good deal less reliable than the officer’s eagle eye? What if the computer suggests you’ve broken the law only because of bad data—garbage in, garbage out?"

A trial judge in the U.S. District Court for the District of Kansas now has a second chance to explore that question in the Justice Department's prosecution of Antonio Esquivel-Rios. He was indicted in early 2011 on possession with intent to distribute methamphetamine. U.S. District Judge Julie Robinson declined to suppress the drug evidence. Esquivel-Rios was convicted at trial.

Gorsuch, sitting with Judge Harris Hartz and Senior Judge Bobby Baldock, noted: "We know almost nothing about the database on which everything hinges. We don’t know how it operates, why it 'usually' fails to yield information about Colorado temporary tags—or, for that matter, even its name."

Among the questions Gorsuch posed: Are temporary tags placed into a registration database in the first place? How long does it take for a temporary tag to show up in the database, anyway?

"How often 'no returns' occur and how often they are attributable to wrongful conduct as opposed to bureaucratic mischief can make all the difference," he wrote.

Federal trial judges, Gorsuch said, need not become "statisticians to evaluate the question of reasonable suspicion in every case. But in a case where the stop is based solely on a database with apparent credibility problems, it's also difficult not to engage with these sorts of questions."

Esquivel-Rios's lawyer, Michael Jackson of Topeka, Kansas, wasn't immediately available for comment. Neither was assistant U.S. attorney James Brown, who argued for the government in the appeals court.

In court papers, Brown argued the "no return" report on the temporary tag was "evidence that the tag either was not registered or fictitious or both, thus indicating that criminal activity may be afoot."

"Under the reasonable suspicion standard," Brown wrote, "this was enough." The officer, Brown said, was "entitled to intervene and investigate" based on the ambiguity over whether the vehicle was properly registered.

The 10th Circuit did not throw out Esquivel-Rios's conviction. The court wants the trial judge to revisit the examination of the traffic stop. (The judge could, ultimately, decide no additional testimony is needed.)

"The reasonable suspicion question is sufficiently close on the record as it currently stands, and sufficiently novel and important, that before issuing a definitive ruling this court would benefit from a district court judgment that takes account of the relevant facts previously overlooked," Gorsuch wrote.

Esquivel-Rios is serving more than 15 years in federal prison.

Contact Mike Scarcella at mscarcella@alm.com.