I posted this as a question to the "Miles to Go" forum of Baen's Bar, but I recall no such mission discussed in any other detail in the books and my quick search of my digital copies has turned up nothing, so I think this was an "offscreen event".
The Wikia entry for Miles's timeline lists this only as an "undated entry", so the odds are it truly has not been covered in the books, nor has LMB clarified it in the FAQ on her site.
Undated adulthood events:
- One mission qualified as an assassination. (Ref: A Civil Campaign, ch 15)
While absence of evidence is not evidence of absence, it's notable that another item on the list, Miles hiring a 10-year-old courier, has a canonical answer that not only are not all references present in other books, but Lois doesn't necessarily know the truth because she hasn't written it yet.
14. When Miles is talking to Nikki in Komarr and mentions a 10-year-old courier, is this supposed to be a reference to Lilly Durona, Jr.?
LMB) The 10 year old courier was a real incident, attached to one of the 35 or so unchronicled Dendarii missions. No, you were not intended to construe her to be Lilly Jr. No, I don't know anything about the rest of the mission at this time. (Lois-Bujold Mailing List, 17 Mar 2003)
And this has been confirmed by Lois:
Nope, It was, as the OP posited, off-screen.
Miles presumably had many missions we did not see, most for Illyan, a few on the side perhaps to keep fleet finances in trim. Ones that go smoothly or don't involve interesting emotional or ethical dilemmas are not good story-bait, though.
Ta, L. Who also sometimes wonders about the Giant Rat of Sumatra and the Politician, the Lighthouse, and the Trained Cormorant.