This secret is slowly being released to the public via the "drip method". Remember NASA is a branch of the Department of Defense. They know more then anybody. Astronauts are the ones to keep an eye on especially Buzz Aldrin. In this short clip I believe he revealed a little to much and had to correct himself. And yes, there is a monolith on Phobos.
C-SPAN: Buzz Aldrin Reveals Existence of Monolith on Mars Moon - YouTube