The two questions that leap into my head regarding a hoard of hidden gold revolve around 'why?'
Why would the illuminati or Chinese or Mongolians etc. hold on to massive amounts of gold for many many generations? I think I'd use it during my lifetime because once I am gone, who cares? Unless they have discovered the elixir of life, I can't see any reason why someone would hold on to (and not use) some store of wealth just in case my great great great great great grand kids need it. My great great great great great grand kids might be little buggers so why should I risk it? Hitler's great great great great great grandparents were probably nice people after all!
Secondly, if I do have a massive amount of gold stored somewhere, why would I dump it onto the market now? I've kept it hidden for the last few hundred years through a lot of strife so my reasons for holding it must be pretty big. And if I did dump it onto the market, how does that help? I will immediately wipe out most of its worth as the gold market is now flooded. It's not as though I could even use it in financial warfare. Can I bring down the USA by revealing another billion tons of gold? Nope, not if they are not on a gold standard.
Maybe I am missing something but even if the extra gold exists, I can't see it affecting us in our lifetimes.