Is it possible that because "treasure" had been found under burial mounds that following on from that it might have been buried under cairns. Then there is natural curiosity..."ere there's a big pile of stones over there...wonder what's under that then?" (that kind of thing)
The description that the cape was found by workmen "quarrying for stones" is a good example of "robbing" of a non-treasure variety, even though it ended up with them finding treasure anyway.