The tranquil scene, a young boy about to bathe at the bend of a river, hangs in the museum of a small town in northern France as if nothing more upsetting than an occasional duster had ever happened to it. Almost a century ago, as the trenches of the first world war snaked ever closer, it was crated up by German soldiers and moved by truck from Douai to a nearby town, then in the last chaotic months sent by barge and train to Brussels together with most of the greatest treasures of the Douai museum.

That painting by Jean-Baptiste Camille Corot, Shepherd Bathing, was returned after the war and is now one of the stars of the museum's major new exhibition of work by the artist and local contemporaries. However more than 250 paintings from the collection which also vanished in 1918 have never been seen since.

"It is truly a mystery why nobody has ever inquired about these missing works before," said Anne Labourdette, the young director of the Musee de la Chartreuse. "The situation with the second world war is very well understood, but with the first world war this is really an untold story. I think in Douai there was so much pain over what happened that for many years people did not want to think about it. And for my predecessors, when some reparation money was paid by Germany perhaps it was more interesting to build a new collection than look for what was lost."

She became director seven years ago and launched the hunt, with researchers and art historians who have recently become interested in the story, uncovering a secret agenda where some Germans wanted French paintings as hostages for the return of German art siezed by Napoleon a century earlier.

She will soon publish the first inventory of the museum collection in 1914, what was taken but returned in 1919, and what vanished forever. She has recovered three pictures, including a lovely Jules Breton which she managed to block being sold in New York, but major missing works include a harvest scene possibly by Rubens, and a mermaid claimed to be by the 15th century German artist Lucas Cranach.

Striking photographs, probably taken by the British in October 1918, show the museum's ransacked galleries, floors littered with empty frames. A solitary landscape still hanging over a door was ironically destroyed in second world war bombings.

Douai is only a few miles from the Belgian border, and in 1914 the Germans took over as a base for troops for the front, organising entertainments and establishing German only shops. It was a point of pride that the museum stayed open. Later the best from Douai and other towns was moved slightly further from the front to Valenciennes to protect it, where the Germans organised a showcase exhibition with an illustrated catalogue which has proved invaluable. Finally it was moved again to Brussels, where after the war the authorities tackled the mammoth task of identifying and returning warehouses full of art.

Labourdette has found reports of losses at each stage, but the saddest fate awaited the pictures judged second rate by the Germans and left behind in Douai – mainly the "petits maitres", local artists including friends and those directly influenced by Corot's luminous paintings which would certainly have hung in the new exhibition.

As the Germans abandoned the town in September 1918, blowing up buildings, bridges, roads and train lines, the museum was looted, its paintings slashed out of their frames and rolled into kit bags or slung into trucks.

"We have no idea where these works are," Labourdette said. "Some must have been destroyed, but not all. Somebody must know where they are. This story is not over."