In WWI, there was no issue with the crown jewels. At the time of the central power capitulation, there were no forces pillaging Germany or Austria.

In WWII they were evacuated or hidden. After years of fighting two fronts, the Axis powers were so afraid of the pillaging Red Army that many relics and crown jewels were evacuated towards the west and handed over to US or British troops, or hidden until these soldiers found them.

One example is the Holy Crown of Hungary, evacuated from German-occupied Budapest to Mattsee, Austria in 1945 where the crown was taken custody by the 86th US infantry division. From there, the crown and jewels were carried to mainland US, were safeguarded in fort Knox and then returned as the cold war neared its end.

http://home.hu.inter.net/jekely/crown.htm

Similar, the Holy Roman Imperial Regalia was taken by Hitler's troops from Austria and deposited into a shelter in Nüremberg, only to be found by US soldiers and returned to Vienna in 1945.

http://www.wiener-schatzkammer.at/imperial-regalia.html

I couldn't find a decent source quick, but the primary Prussian Crown (Wilhelm II.'s) was walled up inside a crypt during WW1 and dug up after the war.