Go visit The Missing Objects Project. It’s a website dedicated to important objects that are known to exist, but which are now missing. They include everything from "Gen. John Cadwalader’s Parlor Sofas" to the Ark of the Covenant. Anyone can suggest a new missing object to be added to the collection. Here’s an email I sent to the Project:
Hello there. I have a proposed Missing Object for your catalog. During the summer of 2006, I visited the Princes Czartoryski Museum in Cracow, Poland. The basis of the museum’s collection is a group of artworks and antiquities collected over centuries by this Polish noble family. The highlight is Leonardo’s The Lady with an Ermine. However, on the opposite wall of this room is a frame containing a picture of a now-missing painting by Raphael. The English-language museum guide describes the painting thus:
The empty frame on the opposite wall stands as a reminder of the missing Portrait of a Youth (ca. 1509-11), by Raphael Santi (1483-1520. This was originally thought to be a self-portrait of Raphael, and later variously described as the portrait of Francesco Maria della Rovere, duke of Urbino, and of Federico Gonzaga, duke of Mantua. The painting was bought about 1800 in Venice from the Giustiniani family by Adam Jerzy Czartoryski and Konstanty Czartoryksi. It was looted by the Germans in 1939 and has not been seen since 1945.
Best of luck with your interesting project!