Washington(CNN) Fridays at the DC federal courthouse are typically days of high alert for the press corps trying to discern what special counsel Robert Mueller's next legal action will be.

But this Friday, court officials went to extreme measures to ensure it was as difficult as possible to figure out what Mueller's team was doing as the US Court of Appeals for the District of Columbia Circuit held a secret and mysterious argument about a grand jury subpoena challenge.

An entire floor of the courthouse was closed to the public and press for more than an hour. During that time, attorneys secretly entered the courthouse to argue before three federal appellate judges over a grand jury subpoena.

The U.S. Court of Appeals for the D.C. Circuit, the E. Barrett Prettyman building - United States Courthouse, April 26, 2013, in Washington D.C.

The mystery of the subpoena appeal appears to date back to early September, when CNN witnessed several lawyers from Mueller's office entering a courtroom to argue against an unknown defense team before a trial-level judge who oversees federal grand jury-related cases.

Clearly, a challenge related to Mueller's grand jury investigation was underway.

Shortly after, that judge, Chief Judge Beryl Howell of the DC District Court, ruled on a case related to a grand jury subpoena, and the losing party attempted to appeal the ruling. The appellate court batted the case back down to Howell, who held a second sealed hearing on October 5. Though CNN was locked out of the courtroom while the arguments took place, the hearing featured the same team from Mueller's office as before, which included top criminal law appellate lawyer Michael Dreeben.

Mueller's office declined to comment on the hearings.

That same day Mueller's team clashed in a sealed courtroom with an unknown opponent, Howell issued another ruling on the same grand jury subpoena challenge she had decided before, sending the losing party back to the appellate court to ask for reconsideration.

Politico a few days later overheard an attorney at the appellate court discussing sealed Mueller court filings -- and the mysterious grand jury challenge got its argument date set before a three-judge panel at the DC Circuit Court of Appeals.

Argument day arrived Friday.

Typically, DC Circuit Court arguments run smoothly, one after another until three cases have been argued publicly, starting at 9:30 am in a large, portrait-lined courtroom on the Fifth Floor of the federal courthouse on Constitution Avenue. But after Circuit judges David Tatel, Thomas Griffith and Stephen William -- who coincidentally has written two books on Russian history -- heard an immigration-related case Friday morning, the courthouse security went into lockdown mode.

Tatel, Griffith and Williams took a brief recess, indicating they'd return to the courtroom shortly.

Then, security officers cleared the appeals courtroom, allowing only about a dozen law clerks working for federal judges to stay behind, including at least one who assists Howell with her cases.

Security guards also cleared the vestibule to the courtroom and checked the coat closet where attorneys coming to listen to arguments stash their belongings. They locked the door leading to the attorneys' lounge on that floor and shooed the more than 20 reporters prowling the hall away from the elevator bank and told them to vacate the nearby stairwells. At one point, even an elevator wouldn't open its doors on the fifth floor.

The entire level of the building on which the appeals court is housed was locked down.

For more than an hour, the press waited, staking out stairwells and exits. The gaggle of law clerks dispersed about an hour after the arguments started, and then silence. No recognizable attorneys were spotted coming in and out of the courtroom or even the building.

No sign that it was Mueller's office. No sign of defense counsel. The courthouse security had ushered the lawyers into and out of the building for their secret hearing completely under cover. The sealed hearing stayed confidential.

And then, about 10 minutes after the court activity appeared to wrap for the day, a black Justice Department car rolled into Mueller's office building, bringing attorneys including Dreeben and Zainab Ahmad back to their home base.