Honestly, I don’t remember. Even Disney Wiki is scant.
Never saw the movie, but I got the book from Scholastic in 4th grade and have reread it several times. (Haven’t read any of the sequels, though.)
There’s much more info in this NYT article.
At least one of the bars has been identifed as dating from the 1960’s so those speculating about Nazi hoards are probably wide of the mark.
There’s also a lot more info in the article about what is likely to happen to it, if the original owners don’t come forward.
I can imagine this scenario: An old couple who has been stashing gold for many years reaches that age when it makes more sense to be living in a retirement home. So they pack up all their stuff and as they’re standing in the doorway on their final day, one of them says “I get the feeling we’re forgetting something. . .”