A woman looking for old crockery in a trash container in the western Austrian town of Zell am See stumbled upon the precious piece in 2004, Salzburg police said.
Apparently she had no idea what she had found. She took the cross home and stashed it behind her couch.
Now experts say the cross could be worth as much as $536,620. A local museum has custody of it, at least for the moment. And whether the trash-foraging woman, who has not been identified, will get so much as a penny for her find has yet to be determined.
She found the cross after a hotel owner who lived in Zell am Zee died, and his apartment was being cleared by relatives, the Austria Press Agency quoted police official Christian Krieg as saying. The woman showed the cross to the niece of the dead man, but the niece didn't want it and allowed the woman to take it, the news agency reported.
Last month, one of the woman's neighbors had an inkling the cross might be something special and took it to a local museum in the village of Leogang.
The curator, Hermann Mayrhofer, alerted police. An investigation revealed that, until World War II, the cross had been part of a Polish art collection belonging to Izabella Elzbieta of Czartoryski Dzialinska.
Before the outbreak of war, Elzbieta tried to hide the piece from the Nazis by concealing it in the cellar of a building in Warsaw. But the Nazis found it in 1941 and later brought it, along with other items from Elzbieta's collection, to a castle in Austria. It is unclear what happened next.
This summer, the cross was taken to Vienna for analysis but it has now been returned to the museum in Leogang. Experts at Vienna's fine arts museum determined that it comes from Limoges, France, and dates to about 1200.
Police said similar pieces have been auctioned for up to $536,620.
In a telephone interview with The Associated Press, Mayrhofer said that he knew straight away that the cross came from Limoges and praised the woman for salvaging it.
"She did something extraordinary," Mayrhofer said.
But he said he couldn't comment on whether the woman would receive any money for her find, adding that the museum would keep the piece until the case is cleared up.
A judge in Zell am See has decided that for now the cross should be kept in the museum security. Mayrhofer said it would soon be included in a special exhibit at the museum.
The London-based Commission for Looted Art in Europe is representing the heirs of the former owner of the cross, police said.