"Freedom is Not Free"

I went to the Korean War Memorial the other day...I was of course moved by the memorial as a whole, but was especially saddened when I read this:





How can so many people be "missing"? Not all of them must have died, I wonder if some of them just went crazy and ended up forgetting who they were...or were some of them kept prisoners for years after the war ended...or they just bother not to tell the government they were alive? This puzzles me...