The Fed may own the entire 8000 tons of US gold.

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https://www.zerohedge.com/markets/rickards-weird-gold-trick-could-end-debt-ceiling-showdown

The Eisenhower administration was up against the debt ceiling. And Congress didn’t raise the debt ceiling in time. Eisenhower and his Treasury secretary realized they couldn’t pay the bills. What happened?

They turned to the weird gold trick to get the money. It turned out that the gold certificate the Treasury gave the Fed in 1934 did not account for all the gold the Treasury had. It did not account for all the gold in the Treasury’s possession.

The Treasury calculated the difference, sent the Fed a new certificate for the difference and said, “Fed, give me the money.” It did. So the government got the money it needed from the Treasury gold until Congress increased the debt ceiling.

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So the Fed has a claim on all the US gold. Not just the gold seized from them. Effectively the Treasury sold all the gold to the Fed and told them "but you can have it"

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https://en.wikipedia.org/wiki/Gold_as_an_investment#Certificates

https://en.wikipedia.org/wiki/Gold_certificate_(United_States)#Series_of_1934_Gold_Certificates;_Modern_usage_by_the_Federal_Reserve_System

As of December 2013 the Federal Reserve reported[10] holding $11.037 billion (face value) of these certificates. The Treasury backs these certificates by holding an equivalent amount of gold at the statutory exchange rate of $42 2/9 per troy ounce of gold, though the Federal Reserve does not have the right to exchange the certificates for gold.

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So it would just take a change slipped into a midnight bill passing through congress to allow the Fed to back the trucks up to Fort Knox and say "Load 'em boys!"