Lots of police would be interested:

The Central Directorate of the Judicial Police (French: Direction centrale de la police judiciaire; DCPJ) is a directorate of the National Police (French: Police nationale) of France with national and territorial competence for investigating and fighting serious crime. If Goring had it taken out of a museum it makes for good copy !

The local / national police where the auction was held and where the item is now

Suggest you start at U.S. National Central Bureau - Interpol


Interpol Washington, the U.S. National Central Bureau, coordinates law enforcement activities between U.S. agencies and their international counterparts.

Agency Details
Website:
U.S. National Central Bureau - Interpol

Contact:
Contact the U.S. National Central Bureau - Interpol-Washington

Main Address:
Interpol Washington
U.S. Department of Justice
Washington, DC 20530-0001

Phone Number:
1-202-616-9000

Government branch:
Executive Department Sub-Office/Agency/Bureau

Parent Agency
U.S. Department of Justice