It’s brave to assume the gold is still there. Nobody checks it; last time they did, a bunch of bars were made of tungsten.
If you’re someone guarding the gold, you’d have to be stupid not to replace it with tungsten. A single bar is a lifetime of wages. It’s not like anyone will ever notice, it’s a reserve that will never be spent.
You can't just walk in and out carrying 12kg bars. Also I don't think you can just buy a bar of tungsten, you gotta smelt it, coat with gold, not trivial. That kind of operation would involve a lot of cooperating people. You also need to convert gold to dollars which might not be as trivial as you think.
So maybe that happens, but it's a lot more complicated. And, of course, there are measures against that happening.
It’s a long time gold bug conspiracy theory. There was a case of a Canadian bar that came from a bank that had a tungsten core but there was a huge outcry about that and it was decades ago.
Chinese gold has been tungsten spiked, fairly often actually, but it’s a known fraud vector there so is broadly audited.
If you’re someone guarding the gold, you’d have to be stupid not to replace it with tungsten. A single bar is a lifetime of wages. It’s not like anyone will ever notice, it’s a reserve that will never be spent.