But her friend did not seem very interested in her discovery, she said.

More time passed, and one day she removed the painting from the wall and examined the back. There she saw several stickers — one from the Perls Gallery in Manhattan, now closed; another from the Musée d’Art Moderne de la Ville in Paris, where it had been exhibited in 1974; and a third from the Richard Feigen Gallery in Manhattan.

She called the Feigen gallery and told someone there about all the information on the labels. Days later, she said, the gallery called back to say it had no record of the painting.

A year or so after that, she said, she told another friend about the painting. “He showed me a Sotheby’s catalog where a Tamayo had sold for $500,000,” she recalled. He also went to the library and came back with a pile of books on the artist. One — a 1974 monograph of his work by Emily Genauer — had her painting on the cover. “I was stunned,” Ms. Gibson said.

She made an appointment to do more research at the Frick Art Reference Library, at the Frick Collection on East 70th Street. A librarian there directed her to the nearby Mary-Anne Martin gallery, which specializes in Latin American art.

She walked three blocks to the gallery, where she says she was told by someone that it was a “famously stolen” painting. “I was in a state of shock,” she said.

Realizing that she might have something very valuable, Ms. Gibson built a false wall in her closet to conceal the painting, carefully wrapping it in old shower curtains. After Googling the artist’s name, she discovered an image of “Three People” at the “Antiques Roadshow” Web site in reference to the “Missing Masterpieces” segment.