"I wonder if they signal each other with their Captain Marvel decoder rings too?"
It would do you wonders to do some research on the intelligence services of the U.S., incl. the DIA, NSA, CIA, etc. You are obviously uninformed and have a low opinion of these groups.
In May of 1998, the NETWORK had a long conversation with CDR Flynn. He related, in great detail, time in captivity with civilian John T. Downey, captured 11/29/52 and held until 03/12/73. He also spent time with civilian Richard Fecteau, captured 11/29/52 and released from China 12/12/71. He said Richard was a Yale Football player who spent his time in captivity jumping rope. Flynn never understood where the energy came from! One day he said, he noticed him eating the "meal of the day," rice gruel. "I had all I could do to eat one bowl," and he was eating SIX! Fecteau went on, he says, to Harvard, married, and became a lawyer. All three speak about once a year. Flynn is still searching the Guinness Book of World Records for the jump rope record Fecteau should have, he says. John Downey has a book relating his story, on the market.