Evidence that the secret case is about Qatar
The suboena was issued in the secret case on July 11, 2018 by the Grand Jury
https://www.law.com/nationallawjournal/2019/02/28/newly-unsealed-orders-in-mueller-mystery-case-reveal-judges-observations/?slreturn=20190222123829
The law firm handling the case has been widely identified as Alston & Bird
https://www.cnn.com/2019/01/09/politics/russian-interests-law-firm-mueller/index.html
A week later, a public relations consultant T. Dean Reed, who works extensively with Alston & Bird, signs a new contract with the Embassy of Qatar (and files a FARA for that).
https://efile.fara.gov/docs/5044-Supplemental-Statement-20181029-32.pdf
One of the Alston & Bird lawyers on the case - Costin - specializes in investment trusts.
https://www.alston.com/en/professionals/c/costin-emily-seymour
This is why Qatar was in the news in 2017. They are knee deep in spygate.