You might be surprised at the humble passenger. Its been a long time so I might be misidentifying the plane but there used to be aircraft like that on standby for classified couriers from lets say maybe the Special Securities Office to some embassy somewhere and back. Sometimes it was a clearance and subject that shouldnt go through the system and sometimes the volume of the exchange made transmission impractical.
Thanks for sharing your expertise and insight.
Im always amazed at the depth of experience we have on FR.