Perhaps you're thinking of L. Q. Washington's statement that Surratt was "unusually mutton-headed" and wasn't told what was in a letter he carried. From James O. Hall's "The Veiled Lady" in vol. 3, no. 6 (2000) of the magazine North and South:
Quote:Surratt checked with a man he knew, Sergeant Harry
Brogden, who was on detail in Richmond to Secretary of State
Benjamin. Until shortly before, Brogden had been in charge of
the Mattox Creek signal corps camp in Westmoreland County.
There was indeed something Surratt could do for the Confederacy.
Brogden took him to see L.Q. Washington at the State
Department. Washington told about this meeting in an interview
with the famous war correspondent, George Alfred
Townsend. The interview was published on January 5,1867:
A professional blockade runner, passing under the
name of Mrs. Howell, came on from New York City
with cipher dispatches, and expected to be joined by a
male escort in Washington. He had been detected and
put in confinement, however, and John Surratt was
substituted for him. Surratt, going by the Surrattsville
route, crossed the river and proceeded to Richmond
where Mr. Washington examined him, and formed the
opinion that he was unusually mutton-headed for one
of his role. He was sent back with a letter to Jacob
Thompson in Canada, "but," added Mr. Washington,
"he never knew what was in that letter."