Captured by the North Vietnamese in 1965, Navy pilot Jeremiah Denton was forced to participate in a propaganda interview to be broadcast in the United States. Pretending to be oppressed by the television lights, he blinked the word “T-O-R-T-U-R-E” in Morse code — alerting U.S. Naval Intelligence for the first time that American prisoners were being tortured.

In his Investigator’s Guide to Steganography (2003), Gregory Kipper notes that captured soldiers would sometimes use hand signals to transmit messages during photo ops; “often, these gestures were airbrushed out by the media.”