Saxophonist Merryl Goldberg went to Moscow with a group, but she did not want to reveal to the Russians any details of a dissident music group they would be visiting. These were the bad old days of the Soviet Union, so anyone talking to Americans might be in deep stuff.

She encoded all of the details into her sheet music.

This article talks about why they were doing it and the coding scheme she invented.

Wonder if the Soviets ever broke the code?

How a Saxophonist Tricked the KGB by Encrypting Secrets in Music – Using a custom encryption scheme based on musical notation, US musicians smuggled information into and out of the USSR.