On 21st August 1911, in Paris, an artist called Louis Béroud was setting up an easel in the Louvre with the intention of sketching the gallery where the ‘Mona Lisa’ resided. He noticed the painting was missing and assumed it had been taken to be photographed (the Louvre photographed their paintings on the roof in natural light).
However, in the name of his art, Béroud insisted on waiting for the ‘Mona Lisa’ and asked the guards about its return. Surprisingly, they knew nothing about it. The pin dropped, alarms bells started ringing, and the frame was found in an empty staircase.
It had taken more than 24 hours for anyone to notice the empty hooks on the wall and the Louvre closed for a week. The nation was outraged but, “the art heist of the century” was pretty rudimentary.
https://impactnottingham.com/2021/10/the-mona-lisa-the-heist-that-made-the-masterpiece/