Reclusive millionaire’s mansion still yielding surprises three years after her death.

A Stradivarius violin that once belonged to Rodolphe Kreutzer has turned up stashed in the closet of reclusive millionaire Huguette Clark. The instrument, which will be auctioned by Christies this week, is expected to fetch as much as $10 million.

The reclusive Clark, a copper heiress who died in 2011 at the age of 104 after a long illness, maintained a sprawling 42-room Fifth Avenue apartment for many years, despite living out her final decades in a nursing home. The instrument, a tiny fraction of her $300 million fortune, was found stuffed into a wardrobe.

Kreutzer (1766-1831), was a virtuoso violinist and composer, best known as the dedicatee of Beethoven’s violin sonata of the same name, although he famously never played it, deeming it incomprehensible. The Stradivarius is being touted as “the one violin that Kreutzer held most special to him and would retain throughout his life,” by the auctioneers.

Clark’s mansion has yielded many treasures over the last three years including a rare pink diamond that fetched $21 million at auction last year. According to a recent biography by Meryl Gordon, Clark was a social butterfly...