The story comes from Harvard Professor James English in his book THE ECONOMY OF PRESTIGE: Prizes, Awards, and the Circulation of Cultural Value
When the Germans began their occupation of Denmark in 1940, the great Danish atomic physicist Niels Bohr was rightly concerned that his Institute of Theoretical Physics in Copenhagen had in recent years become a harbor not only for leading German physicists but for the Nobel medals of some of the German laureates who had taken refuge there--in particular James Franck and Max von Laue. Since winners' names are engraved on Nobel medals, these were particularly dangerous pieces of evidence against Bohr's colleagues. Anticipating a raid on his facilities, Bohr managed, just hours before the Nazis arrived, to have the medals dissolved--a more demanding chemical operation than one might think, and not one for which a theoretical physics lab is well suited. The resultant jars of liquid meant nothing to the military officials conducting the search, the institute was spared, and after the war the gold was recovered from its solution and returned to the Nobel Foundation, which had new medals cast for the two laureates."
--James F. English, The Economy of Prestige, p. 157