Hypothetically speaking of course, what would be the course of action for a FI if it discovered that the existence of a Summons to Trustee was mentioned in the narrative of a SAR filed?

I don't understand? Why would you need to take any action, It would not be prohibited. It has nothing to do with a grand jury proceeding. Those involve civil actions don't they?

Yikes, long week. I think the thought process here was that the Summons was the basis for filing and noted in the narrative; not simply that the receipt of the Summons was noted in general.

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.

Right, I think that's what I was held up on; the "hypothetical" narrative in question simply states that after further investigation it was discovered that there was a subpoena issued for the subject, which is approximately when the activity noted in within the SAR began.