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

On Tuesday, a gardener clearing ivy from Ricci-Oddi modern art gallery in the northern Italian city of Piacenza came across a metal panel in the wall and forced it open to look inside the cavity between the external and internal gallery walls. 

“I found this box inside a black sack and at first I thought it was trash, but then I called my superiors right away,” the gardner told Italian media.

The gallery will not make 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.

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

Jonathan Papamerenghi, a member of the Piacenza council with responsibility for culture, said it could be possible that the painting had been left in the wall by thieves who later 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 reported on the morning of February 22, 1997, but police believed it had been stolen three days earlier.

Investigators initially suspected an inside job but had no luck finding the suspects.

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 later discovered.

The painting 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 known “double” Klimt.