The conspiracy theorists [who are often right, I'm not knocking them] say that all there is in the vault are post-it notes with an illegible signature on an IOU.
Musk and Big Balz don't need to walk into the vault and do a physical audit. Would the crazies believe it anyway, would they ask if he did a core sample on every bar? That's not his shtick.
Assuming that Bob the guard didn't walk out with bars in his pocket EVERYTHING has a paper trail and there guys have shown they can do that.