The gold nugget case

Discovered in a field ot the south of Angers by a walker in 2001, a gold nugget, since purchased by that city, is on the center of a clash. On one side the public new owner, Angers, and on the other, the wife of the former chairman of "L'oreal", one on the world's largest companies of cosmetics who claim that the nugget was stolen to her mother.
But if that is true, how such a value could lay in a middle of a field in 2001 while the disappearance was reported to the police in 2008?
The nugget, whose weigh is 57 gram, was first sold par the walker to a group of three collectors for 2 700 € then sold by these men in 15 000 € to Angers.
Many people would be interested by the purchase of the field, in cas of a gold mine...