She called herself “Renoir Girl,” refusing to reveal her identity and offering few details about her biography. She lived in Northern Virginia, once taught in Washington area schools and was well-known in her community.

What made her irresistible to reporters wasn’t who she was, but what she said she’d found: A bona-fide painting by Pierre-Auguste Renoir in a $7 box of trinkets purchased at a West Virginia flea market.

The story generated worldwide attention and, for a time, promised to produce a six-figure windfall at auction for its accidental owner. But late last year, the FBI seized the painting, called “On the Shore of the Seine,” after the Baltimore Museum of Art learned it had been stolen in 1951....