Two tour guides at the historic castle built by media mogul William Randolph Hearst have helped unravel the mystery of attribution behind a 17th century artwork in the museum's collection.

On a tour, a ray of sunshine illuminated a previously unnoticed or overlooked monogram and inscription on a massive painting in the Assembly Room of Hearst Castle’s 68,500-square-foot main house, La Casa Grande, in San Luis Obispo, Calif. The tour guides investigated the inscription, which had long gone unseen in the dimly lit room, even after conservation.

For decades displayed as a work by an unknown artist, the circa-1690 painting of the Annunciation, has now been confirmed to be the work of Spanish artist Bartolomé Pérez de la Dehesa, thanks to the eagle-eyed guides, Carson Cargill and Laurel Rodger.

“This is a major new discovery for the oeuvre of Pérez,” said Museum Director Mary Levkoff.