Mentioning a grand jury subpoena or a national security letter is verboten.
I'm familiar with a "summons" issued by the IRS or other governmental agencies, but generally the term used by civil and criminal courts is a "subpoena." You are not required to disclose what served as the impetus for the enhanced due diligence that resulted in a SAR filing, but it might be helpful to law enforcement to know they were out there.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.