Art gallery owner ­Alexander Acevedo was living the art hunter’s dream.

Only one other person at the auction in upstate New York a few years ago seemed interested in the handsome 18th-century portrait of British earl William ­Ponsonby by an unknown artist. The auction catalog’s suggested value: $1,500 to $2,500.

But Acevedo instantly recognized it as the work of none other than John Singleton Copley, the famous painter whose statue presides over Boston’s Copley Square, potentially making the painting worth more than $1 million.Acevedo managed to snag the portrait for just $85,000.

When Acevedo researched the painting, his heart sank. It was stolen from Harvard University’s storage in 1971. Acevedo promptly ­returned the stolen work to the ­auctioneers, and it is now back at ­Harvard.

Art detectives say long-lost works like the Copley are increasingly turning up after going missing for ­decades, thanks in large part to readily available information on the Internet or in electronic databases. The trend is feeding hopes of art fans that the prized pieces taken from the Isabella Stewart Gardner Museum 23 years ago could eventually surface as well.

Though the vast majority of missing artwork is never recovered, stolen items are often discovered when they change hands, sometimes many years later, when brokers and buyers research the pieces online and through databases, according to brokers and others in the business.

The Gardner theft, the costliest museum heist in history, remains one of Boston’s most enduring mysteries. Thieves took 13 items worth roughly half a billion dollars, including Rembrandts, a Vermeer, and a portrait by Edouard Manet.

In March, the FBI said it ­believed it knows who committed the crime and traced some of the art to Philadelphia, where it was offered for sale. But the bureau said it is unsure where the art is today.

Though no hard data is available on how recovery rates have changed, many art detectives believe that dealers and collectors are increasingly spotting stolen items as information becomes more widely distributed on the Internet and in searchable databases, such as the global Art Loss Register, started by the insurance industry in 1991, and the FBI’s National Stolen Art File, which was put online three years ago.

In the past, art brokers had to consult catalogues, which were often out of date and less comprehensive. In addition, law enforce­ment can now instantly blast alerts about stolen works around the world. Many auctions are also advertised online, allowing more enthusiasts to see the items up for sale.

In January, a Swedish museum recovered a Matisse painting that was stolen 26 years ago when a thief forced his way into the Moderna Museet in Stockholm with a sledgehammer. The painting turned up after a British art dealer who had been asked to sell the work checked the Art Loss Register database.

Six years ago, New York art dealer Lawrence Steigrad discovered that a valuable painting he was offered had been stolen from a Concord home in 1976. Even before he saw the 1764 painting by Swiss artist Angelica Kauffman, Steigrad said he noticed the piece was “reported missing” in an old art journal, and he confirmed that it was stolen through the Art Loss Register.

Still, databases are only valuable if they are put to use. Some buyers and dealers do not bother to check them, either ­because they trust the seller or do not want to pay the research costs. (The Art Loss Register typically charges between $32 and $195 to check an item.) And not every theft victim ­reports such losses.

Overall, only about 7 percent of the more than 7,500 items listed in the FBI database have been recovered since the list was started on paper in 1979. Both the Art Loss Register and Interpol, an inter­national group of police agencies, estimate that 5 to 10 percent of works on their lists are eventually recovered.

While museum heists garner the most headlines, most of the art works listed in the Art Loss Register was taken from private homes and collections and are worth less than $10,000 each. The most common artist in the database is Spanish master Pablo Picasso, one of the world’s most prolific painters and sculptors, who created more than 800 of the missing items.

In many cases, art surfaces only when someone inherits it from a family member or friend who has died and then brings the piece to an art dealer who discovers that it was stolen. Sometimes it can take multiple sales before someone notices that a work was stolen, particularly for less expensive items.

Harvard alone has 300 missing items, including a Chinese green jade container from the Ch’ing dynasty in the late 18th or early 19th century that was discovered missing from an ­exhibit in 1979, according to the Art Loss Register.

Acevedo, who bought the Copley, said he realized that the work might be stolen hours ­after the auction when he spotted references to the painting being in the Fogg Art Museum’s collection at Harvard in the 1960s or 1970s.

Though museums occasionally sell minor pieces to prune their collections, Acevedo said he did not think the Copley was the type of work Harvard would sell. A museum curator promptly confirmed his worst fears: It was indeed stolen. Though the auctioneer refunded what he paid for the work, Acevedo watched his potential profit evaporate.

Chris Sabian is a portrait artist with http://www.kutefineart.com and owner of http://www.paragonprints.co.uk