The 13-year-old, Milly Hardwick, said that she, her father and her grandfather had been out in a field with metal detectors for several hours on a Sunday in Royston, England, and had not found a single item. Then, just after a lunch of sandwiches and cookies, they tried a different part of the field, where an organized dig was taking place. After about 20 minutes of searching, Milly said she heard the high-pitched beeping noise — “a lovely-sounding signal” — that a possible find.