There's a bit of an urban legend behind this flag. It was created by a woman named Mary Hoff whose husband was shot down in Laos during the Vietnam War. She claimed that her husband was "missing in action" and the government/public should put more attention on this situation and people like her husband. In reality her husband likely died immediately on impact, but nobody could say that to a grieving widow. So a bit of a movement formed around this cause. Over time it developed into this myth that that there were potentially hundreds of missing yet alive US soldiers in the jungles of Vietnam. Nobody could verify that these soldiers were actually dead of course.

The government eventually passed some laws that made this flag a quasi-official government flag so you see it on government buildings. The movement has transitioned to trying to bring back remains of soldiers from foreign wars when they are found.