I think they hid the Gold in plain sight, and the secret just got forgotten about over the years, and Rapaire just stumbled across it.

It's hard to go through a detective book of a certain era without the gold having been melted down, copper plated and turned into those old heavy pans that you don't use any more, the sash window weights being the family silver.

Of course, this is easier in places where the written history goes back more than two centuries, but you've got to work with what you have....