John Singleton Copley (1738-1815) Painting of William Ponsonby, Second Earl of Bessborough, 1790

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 hand, sometimes many years later, when brokers and buyers research the pieces online and through databases, according to brokers and others in the business.

“We’ve got recoveries happening every week,” said ­Christopher A. Marinello, an ­attorney for the Art Loss Register of London, which maintains an international database of more than 360,000 stolen, looted, disputed, or missing works around the world, including 1,000 from Massachusetts and hundreds of pieces from ­Harvard alone.

“It’s not that unusual to find artwork that has been lost for more than a quarter of a century,” Marinello said. “The valuable pieces either are recovered right away, or they go underground for a generation.”

In 1990, two thieves dressed as Boston police officers told the museum night guards on duty that they were responding to a call. The thieves passed the sole security door. There was just one alarm button at the time; only motion detectors traced their movements. There were no cameras. A mere 81 minutes later, they were in possession of the masterpieces worth, today, half a billion dollars. The investigation is ongoing. 

The new building could have been a fortress. But that would have made the theft the focal point of how we would perceive the museum. Instead, the colorless glass entry, the brick walls, even the enclosed corridor that passes from the new building through a grove of trees into Gardner’s historic courtyard serve as practical access controls. There are no doors for the public to the original Gardner mansion. A thief would now have to walk through a transparent glass tunnel, into the new building, and out a security door for the easiest exit. Though counterintuitive, its openness makes it more secure. 

While the museum is watched by hundreds of cameras, the new structure is designed to relieve some of the stress from Gardner’s old home by shifting the burdens of exit and entry to the much more modern and secure building. “There is simply no place in the museum where a thief can just grab art and get outside,’’ Anthony Amore, the head of museum security and author of “Stealing Rembrandts,’’ said.