The use of US court-sanctioned wiretaps is on the rise. According to the most recent figures available, the number of taps increased 17 percent last year over the previous year.

The latest federal Wiretap Report shows there were 4,148 non national security related wiretaps authorized in 2015. Not a single application was denied, the report notes. Of that total, 3,297 were granted an extension over the original time period authorized by the warrant.

Given all the access, just when should the cops hang up on the call they're bugging? A federal appeals court recently provided the answer—introducing the "plain hearing" principle.

This guidance concerns when the cops know, or reasonably know, that the speakers on a call are outside the scope of the original warrant. The plain hearing principle is similar to the well-known "plain view" doctrine, which allows authorities to seize physical evidence unrelated to a warrant if it's in plain view of the police during a search.

According to the 9th US Circuit Court of Appeals, this announced plain hearing doctrine means: (PDF)

The Fourth Amendment provides an exception to the warrant or probable cause requirement when police see contraband in "plain view." We adopt a similar principle today and hold that the police may use evidence obtained in "plain hearing" when they overhear speakers unrelated to the target conspiracy while listening to a valid wiretap, without having complied with the Wiretap Act requirements of probable cause and necessity as to those specific speakers. However, the agents must discontinue monitoring the wiretap once they know or reasonably should know that the phone calls only involved speakers outside the target conspiracy.

With that theory in hand, the San Francisco-based appeals court, in a 2-1 ruling, overturned a lower court judge who refused to suppress evidence that drug agents obtained on a suspected cocaine dealer named Michael Carey. As fate would have it, Carey was unconnected to the original drug conspiracy the authorities were investigating, according to court documents. But the authorities continued monitoring Carey's communications under the same warrant using the same phone number authorized by the original warrant. During that monitoring, the authorities learned of a different drug conspiracy allegedly involving Carey. (Again, the initial warrant did not authorize the interception of communications of that alleged drug conspiracy.)

Carey eventually moved to suppress evidence obtained via the wiretap, arguing that the authorities breached the Wiretap Act because they never applied for a wiretap warrant for him or his co-conspirators. The Southern California federal judge presiding over the case originally ruled that the warrant was all that was needed. Carey appealed, again arguing that any evidence obtained via the wiretap was inadmissible because the authorities did not have a lawful warrant to monitor his communications for seven days. The authorities, however, maintained that the agents had the lawful right to continue monitoring the wiretap even after realizing they were not listening to conversation about the targeted conspiracy.

The appeals court said Carey's case asks a "novel" question of US law.

But what happens when a wiretap that is valid at its inception is later used to listen to someone who is not involved in the conspiracy under surveillance? It is that novel question to which we turn our attention.

In this instance, the 2010 valid wiretap order, the court majority wrote, "could not authorize surveillance of an unknown conspiracy because the statute requires agents to demonstrate probable cause and necessity to procure a wiretap order."

The majority concluded Wednesday that once the authorities "know or should know they are listening to conversations outside the scope of the wiretap order, they must discontinue monitoring the wiretap until they secure a new wiretap order, if possible."

The decision is not a complete victory for the defendant, however. The majority ordered the case back to the lower court to develop a record on when the authorities realized they were intercepting calls unrelated to the original conspiracy, which court records call the "Escamilla conspiracy" (named after Ignacio Escamilla Estrada, the original drug suspect).

"The record does not indicate what evidence was obtained before the agents knew or should have known that they were listening to calls outside of the Escamilla conspiracy," the majority ruled. "It is unclear how much of the government’s wiretap evidence may fall outside of the 'plain hearing' doctrine."

The opinion was written by Judge Ronald Gould, a President Bill Clinton appointee. Gould was joined by Judge William Fletcher, also a Clinton appointee.

In dissent, Judge Alex Kozinski, a President Ronald Reagan appointee, chided the majority. He wrote that the evidence of drug dealing should remain valid. The officers, he wrote, acted properly because they could have reasonably believed Carey was part of the original, targeted drug conspiracy.

While agents eventually realized that Escamilla wasn’t using the phone, the wiretap order also permitted them to intercept conversations of Escamilla’s unknown co-conspirators. The agents could have reasonably believed that Escamilla had passed the phone to a confederate.

No new court date for Carey has been set at this time.