The theft of Sisi's Star from Schönbrunn Palace
Vienna, Austria, 1998
This case is a work of fiction. The characters, names, places and events in the game do not portray real people. The account below is a factual summary, based on public sources, of the case that served as inspiration.
In June 1998, one of the diamond and pearl stars that Empress Elisabeth of Austria, known as Sisi, wore in her hair disappeared from a display case at Schönbrunn Palace in Vienna, with no broken glass and no alarm set off. A replica was left in place of the jewel, and the loss was only noticed later.
The piece resurfaced in 2007, in Canada, when a professional thief arrested for other crimes revealed he had kept it for almost ten years. He said he had swapped the star for a copy from the gift shop during his visit.
Sources
In the game

Interrupted Waltz
Palais Lindenau, Vienna, December 31, 12:10 am. The Tiara of the Seven Stars, belonging to Countess Adelheid von Lindenau, has vanished from the display case in the Golden Hall, where it was waiting for the 12:30 am charity auction.
The game borrows the imperial jewel that vanishes from a display case in a Vienna palace with no sign of forced entry. Characters, perpetrator and outcome are fiction.