If they are commonplace why is there only one in a museum - they may have been in circulation between monasteries a millennium ago but genuine ones certainly are not common now. I think you've probably made the scenario a lot worse by suggesting they are lying around all over the place if you want to take your metal detector out and have a go.
Anyone who has watched the series will have seen in the opening sequence a medieval monk burying the artefact at the spot where it was found so your last paragraph is unlikely.
My good friend K, who liked the series a lot, says the series is only superficially about finding 'treasure' rather instead about finding the hidden treasure of everyday relationships. So on that note I'm bowing out.