The claim: Detention center, possibly located near Salt River, in restricted area of base. Local patriots advise that black Special Forces & UN gray helicopters are occasionally seen in area.
What it really is: There are lots of places at Fort Knox that are restricted (such as the United States Bullion Depository) and places open to the public (like the General George Patton Museum of Leadership).
As for the claim of a detention center being located near Salt River, I've looked along the river using Google Maps, and I can't find anything that looks like a prison camp, just some roads, some cleared out areas for training, and a few small buildings.
As for the black Special Forces and UN gray helicopters claim, these claims are not backed up by reliable sources, and is most likely a hoax.