I once rented a workshop that had previously been the home of a small jewelry making shop. Before moving my gear in, I vacuumed and mopped every surface and ran everything through a makeshift sluice box.

Recovered nearly 2oz of (powder) gold and several dozen gemstones.

Of course I immediately cleaned out the air ducts, all the way to the plenum, and dug up the ground near the entry - recovering another oz or so that I suspect was simply tracked out by the previous tenant on his shoes over the years.

I wonder if these guys have found a similar angle. Gold tends to accumulate in the lowest cracks.