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.

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