It’s hardly a place you would expect to find a $1 million painting.But one March morning four years ago, Elizabeth Gibson was on her way to get coffee, as usual, when she spotted a large and colorful abstract canvas nestled between two big garbage bags in front of the Alexandria, an apartment building on the northwest corner of Broadway and 72nd Street in Manhattan. “I had a real debate with myself,” said Ms. Gibson, a writer and self-professed Dumpster diver. “I almost left it there because it was so big, and I kept thinking to myself, ‘Why are you taking this back to your crammed apartment?’” But, she said, she felt she simply had to have the 38-by-51-inch painting, because “it had a strange power.” Art experts would agree with her. As it turns out, the painting was “Three People,” a 1970 canvas by the celebrated 20th-century Mexican artist Rufino Tamayo that was stolen 20 years ago and is the subject of an F.B.I. investigation. Experts say the painting — a largely abstract depiction of a man, a woman and an androgynous figure in vibrant purples, oranges and yellows — is in miraculously good condition and worth about $1 million. On Nov. 20 it is to go on the block at Sotheby’s as one of the highlights of a Latin American art auction. Ms. Gibson said she did not suspect that the painting had any commercial value when she found it. “I am not a modern-art aficionado,” she said. “It was so overpowering, yet it had a cheap frame.” The painting had been missing for so long that the owners, a married couple whom Sotheby’s would not identify, had long since given up hope of ever seeing it again. The husband, a Houston collector and businessman, had purchased “Three People” at a Sotheby’s auction in 1977 as a birthday present for his wife. He paid $55,000 for it. Ten years later, when the couple were in the midst of moving from a house to an apartment in Houston, they put the painting into storage at a local warehouse. It was there that it disappeared. The couple reported the theft to the local and federal authorities, and an image was posted on the databases of the International Foundation for Art Research and the Art Loss Register. They also offered a $15,000 reward to anyone who could help them recover it. But no credible leads surfaced.The couple later moved to South America, and the husband died. It is his widow who is putting the painting on the market. How “Three People” got from a Houston warehouse 20 years ago to the streets of New York remains a mystery. The painting’s disappearance so troubled August Uribe, an expert at Sotheby’s, that he volunteered to appear on “Antiques Roadshow” in a “Missing Masterpieces” segment in May 2005. Ms. Gibson had hung the painting in her living room, but remained curious about it. She had gone back to the Alexandria the day after taking it home and asked the doormen there if anyone could tell her who had put it on the street. “No one remembered anything,” she said. “All they said was that 20 minutes after I took it, the garbage truck arrived. This was truly an appointment with destiny.” It took three years for her to realize that she possessed a stolen painting. A few months after she hung it in her apartment, she said, she called a friend who had worked at an auction house and described the painting to him. “He asked me if it had a signature,” she recalled. It did. In the upper right-hand corner the artist had signed it “Tamayo 0-70.” 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. Searching the Web in May, she discovered that the episode would be rebroadcast the next day in Baltimore. She traveled to Baltimore by bus and checked into a hotel to watch the segment. “It was very nail-biting, but the moment I saw it, I knew it was my painting,” she said. Upon returning to New York, she immediately called Sotheby’s and made an appointment to see Mr. Uribe. “Just call me a Mystery Woman,” she says she told his office, not wanting to reveal her story until she was face to face with Mr. Uribe. She asked a minister from her church, the First Church of Religious Science, to accompany her and introduced herself as Mrs. Green. “I asked her to describe the painting,” Mr. Uribe recalled. “And when she said it had a sandy surface, I knew it was the painting.” (Tamayo frequently ground sand and marble into his paint.) She also told Mr. Uribe about the stickers on the back, which offered further confirmation that she had the real thing. Mr. Uribe visited Ms. Gibson’s Upper West Side apartment, and she began dismantling the false wall. “I saw only a corner of the canvas, yet I knew it was the painting,” he said. “The colors and surface were unique to Tamayo.” Ms. Gibson will receive the promised $15,000 reward from the seller, as well as a smaller finder’s fee from Sotheby’s, which the auction house declined to disclose.Sotheby’s informed the F.B.I. that “Three People” had been found. James Wynne, the agent in charge of the case, said that because a criminal investigation was continuing, he could not discuss whether the agency had any clues to who stole the work years ago.“Finding a $1 million painting in the garbage is very unusual,” Mr. Wynne said. “It’s a real New York story.”