A strange case is working its way through the DC Circuit court which appears to involve special counsel Robert Mueller and a mystery grand jury witness, according to Politico.

It's unclear exactly what the two sides are fighting over, but the case appears to resemble a separate legal battle involving an associate of Trump ally Roger Stone, Andrew Miller, who is fighting a Mueller subpoena. Miller's lawyers are using the case, slated to be argued at the D.C. Circuit Court of Appeals early next month, to mount a broad legal assault on Mueller's authority as special counsel. In the more shadowy case, which involves an unknown person summoned before a grand jury this summer, the D.C. Circuit on Monday set a separate round of arguments for Dec. 14. The case traveled in recent months from U.S. District Court Chief Judge Beryl Howell to the U.S. Court of Appeals for the D.C. Circuit, back down to Howell and back up again to the appeals court with most details shrouded in secrecy, another indication that much of Mueller's activity is taking place behind the scenes and is rarely glimpsed by the press or public. -Politico

There is no specific mention of Mueller or his team on the two dockets for the mystery grand jury fight, however earlier this month, a Politico reporter staking out the court clerk's office on the due date for a key filing in the dispute observed a man request a copy of the special counsel's latest sealed filing in order to craft a response. "The individual who asked for the secret filing declined to identify himself or his client and replied “I’m OK” when offered a reporter’s business card to remain in touch," Politico reports.

Three hours later, a sealed response was submitted in the grand-jury dispute. Meanwhile on Wednesday "another detail emerged" which suggests that the secret legal battle is with Mueller; a Trump-appointed Judge with a potential conflict of interest in the Russia case bowed out of a decision in the case.

The first appeal appears to have been rejected by a D.C. Circuit panel as premature. The witness's lawyers asked the full bench of the appeals court to review that decision but a notation in court files says only nine of the court's 10 active judges participated. Bowing out was Judge Greg Katsas, the court's only member appointed by President Donald Trump. Katsas served as a deputy White House counsel before Trump tapped him for the powerful D.C. Circuit last year. At Katsas's confirmation hearing, he acknowledged working on some issues related to the Russia investigation and signaled he would take a broad view of his recusal obligations stemming from that work. "In cases of doubt, I would probably err on the side of recusal," Katsas told senators last October. A spokesman for Mueller's office, Peter Carr, declined to comment on the litigation. -Politico

Trump's personal attorney Jay Sekulow said he has have "no idea" about the case when shown the docket. Roger Stone's attorneys similarly said that they didn't know of anyone challenging a Mueller subpoena aside from Miller, while other attorneys representing other witnesses in the Russia probe say they are unaware of who might be trading barbs with Mueller's team.

So the mystery witness remains a mystery - while their legal team filed a joint motion with the special counsel's office earlier this month asking the court of appeals to expedite resolution of the dispute.

What is known about the mostly sealed dispute based on the DC Circuit's docket is that the case was brought on August 16, and Judge Howell ruled on it September 19, with an initial appeal filed five days later.

"The bottom line is the most likely scenario is someone filed a motion to quash or otherwise resisted a grand jury subpoena, and the judge issued an order denying that and saying the witness needs to testify," said Gibson Dunn & Crutcher attorney Ted Boutrous, whose firm has handled grand jury-related litigation for media organizations and journalists.

It's unclear whether the case the appeals court has agreed to hear in December involves an assertion of attorney-client privilege or some other privilege, is framed as a broader attack on Mueller's authority, or perhaps advances both sets of arguments. "It's very hard to tell from this docket," Boutrous said. The grand jury cases pose a threat to Mueller's investigation because they can serve as vehicles to get questions of his authority and legal legitimacy before appellate judges relatively quickly. Such questions have also been raised by defendants in some of Mueller's criminal cases, but all the human defendants who have set foot in a courtroom have ultimately decided to plead guilty and drop any challenges to the special counsel's authority or tactics. -Politico

As Politico notes, sealed mystery cases involving grand jury matters and independent special counsel investigations are fairly common. "A 1997 conflict-of-interest investigation into AmeriCorps chief Eli Segal’s fundraising activities was conducted under seal from its start," for example. "And a final report remains out of public view involving another Clinton-era probe into Labor Secretary Alexis Herman and influence peddling accusations."

"This can get a step or two weirder than it already is," said one attorney representing a senior Trump staffer in the Russia investigation, recalling a case from a prior matter where he wasn't even allowed access to a judge's opinion because it was under seal.

"It could be anyone who’s been subpoenaed by the special counsel for anything," they attorney added.