It is one of the more baffling story lines of Donald J. Trump’s second term. The president has said he wants to personally visit Fort Knox to ensure that no one has stolen the government-owned gold bars that are stored there. Mr. Trump has not explained why any gold might be missing from the nation’s heavily guarded reserves. His own Treasury secretary, Scott Bessent, has insisted that there is no reason to worry. “All the gold is there,” Mr. Bessent emphatically told Bloomberg in February, at one point looking directly into a camera and addressing the American people. Mr. Trump’s...