As young marines, Peter MacDonald and Thomas Begay transmitted top secret military messages using their native language. It was an undercover mission that changed U.S. intelligence operations forever.

For example, CD9, P911, MOS, and POS all mean “parents are nearby.” They’re used to quickly change the topic or switch to a different app.

The anonymous notes turned darker and more harassing, according to a new Netflix documentary about the case, “Unknown Number: ...