Reddit reviews Defiant: The POWs Who Endured Vietnam's Most Infamous Prison

I looked around for an answer to this, and the only thing I've been able to find is an excerpt from a book called Defiant: The POWs Who Endured Vietnam's Most Infamous Prison suggesting that U.S. intelligence agencies picked up on it at least:

> “When U.S. intelligence agencies reviewed the video before it was aired, they appreciated more than just the commander’s courageous statement. He had also covertly relayed information about the conditions in Hanoi. The blinks that seemed so strange to television viewers were, in fact, very deliberate. Unbeknown to his captors, Jerry had blinked in Morse code.”

Also read a cool tidbit from a WaPo article

>In his memoir, Adm. Denton recalled devising a communication system for the inmates involving coughs, sniffs and sneezes. Such noises, because of the men’s poor health, was not readily discerned by the North Vietnamese and allowed the POWs to maintain a spirit-sustaining sense of community