THEY HAVE NOT BEEN FOUND. There are pictures of "black boxes" posted everywhere at the site to enable the workers to identify them, no matter what state of damage the boxes may be in. We're talking about a lot of people down there. Press, too. It would be VERY BIG NEWS if they were found, and nobody could keep it quiet.
But I have the feeling you think it was be an easy matter for the government to silence hundreds of civilians who would otherwise spread the news. Do they replace the Fire & Rescue folks with clones, or just use hypnosis and drugs?