Italian police are investigating after a painting believed to be a Gustav Klimt stolen almost 23 years ago was discovered hidden in a wall of the gallery where it had previously been on display.

The location of Portrait of a Lady, one of the world’s most sought-after missing artworks, has been a mystery since it was stolen in 1997.

On Tuesday, a gardener clearing up ivy on an exterior wall of the Ricci Oddi modern art gallery, in the northern city of Piacenza, discovered a metal panel which, when opened, revealed a cavity with a painting in a bag.

The gallery is not making an official announcement until the painting’s authenticity is confirmed, but according to sources quoted by the local newspaper, Piacenza Sera, an initial expert inspection indicated the painting was the 1917 Klimt, a later work by the Austrian art nouveau painter.

The director of the gallery, Massimo Ferrari, told the Italian newspaper La Libertà that “the stamps and wax behind the picture are original”.

Jonathan Papamerenghi, a member of the Piacenza council with responsibility for culture, did not exclude the possibility that the painting had been left in the wall by thieves who wanted to return it.

“It is very strange, because, immediately after the theft, every single inch of the gallery and garden was checked with a fine-tooth comb,” he told La Repubblica. “The strangest thing is that the painting is in excellent condition. It does not seem like it has been locked under a trapdoor for 22 years.”

The theft of Portrait of a Lady was discovered on the morning of 22 February 1997 but police believed it had been removed three days earlier. Investigators at the time suspected an inside job. The investigation was reopened in 2016 following the discovery of DNA traces of a thief on the painting’s abandoned frame.

Police believe the thieves used a fishing line to hook the masterpiece off the wall and haul it up through an open skylight to the roof of the gallery, where the frame was discarded.

The Klimt is considered particularly important because shortly before its disappearance an art student realised it had been painted over another work previously believed lost – a portrait of a young lady that had not been seen since 1912 –making it the only “double” Klimt known to the art world.

“If the findings confirm the authenticity of the painting, it would be a sensational discovery and we would be ready to exhibit it in the gallery as early as January,” Papamerenghi said. “We are talking about the most sought-after stolen painting in the world after Caravaggio’s Nativity with St Francis and St Lawrence.”

Stolen in 1969 from the Oratory of San Lorenzo, in Palermo, the disappearance of Caravaggio’s Nativity with St Francis and St Lawrence is considered one of the greatest mysteries in art history. In a video interview filmed in 2001 but locked in a drawer and revealed exclusively to the Guardian in October, the oratory priest said it had been kept in the home of a powerful mafia boss who sliced off a piece of the canvas in order to convince the Catholic church to negotiate for its return.

According to the most recent stolen artworks bulletin, issued by the carabinieri, 8,405 items have gone missing in Italy in the last year alone. These include archaeological artefacts, ancient weapons and medieval texts. Statues and paintings have been taken from churches, which often have no security systems. More than 1m works are still missing.