There are many rumors of what happened to the Nobel medals of three Nobel Laureates in Physics during World War II: the medals of the Germans Max von Laue (1914) and James Franck (1925), and of the Dane Niels Bohr (1922).
Professor Bohr's Institute of Theoretical Physics in Copenhagen had been a refuge for German Jewish physists since 1933. Max von Laue and James Franck had deposited their medals there to keep them from being confiscated by the German authorities. After the occupation of Denmark in April 1940, the medals were Bohr's first concern, according to the Hungarian chemist George de Hevesy (also of Jewish origin and a 1943 Nobel Laureate in Chemistry), who worked at the institute.
In Hitler's Germany it was almost a capital offense to send gold out of the country. Since the names of the Laureates were engraved on the medals, their discovery by the invading forces would have had very serious consequences. To quote George de Hevesy, who talks about von Laue's medal: "I suggested that we should bury the medal, but Bohr did not like this idea as the medal might be unearthed. I decided to dissolve it. While the invading forces marched in the streets of Copenhagen, I was busy dissolving Laue's and also James Franck's medals. After the war, the gold was recovered and the Nobel Foundation generously presented Laue and Frank with new Nobel medals" (struck from the original gold).
de Hevesy wrote to von Laue after the war that the task of dissolving the medals had not been easy, as gold is "exceedingly unreactive and difficult to dissolve." The Nazis occupied Bohr's institute and searched it very carefully but they did not find anything. The medals quietly waited out the war on a shelf in the Institute in a solution of aqua regia.
de Hevesy did not mention Niels Bohr's own Nobel medal but documents in the Niels Bohr Archive in Copenhagen show that Niels Bohr's Nobel medal, as well as the Nobel medal of the 1920 Danish Laureate in Physiology or Medicine, August Krogh, had already been donated to an auction held on March 12, 1940 for the benefit of the Fund for Finnish Relief. The medals were bought by an anonymous buyer and donated to the Danish Historical Museum in Fredriksborg, where they are still kept.
Regarding the Nobel medals of von Laue and Franck, the Niels Bohr Archive has a letter from Niels Bohr dated January 24, 1950, about the delivery of gold to the Royal Swedish Academy of Sciences in Stockholm relating to these two medals. The proceedings of the Nobel Foundation on February 28, 1952, mention that Professor Franck received his recoined medal at a ceremony at the University of Chicago on January 31, 1952.