When Noah Charney wrote a book about the fate of the world's most frequently stolen artwork, The Ghent Altarpiece by Jan van Eyck, he told the story of the Monuments Men and the race to save Europe's art treasures that had been looted by the Nazis. This included The Ghent Altarpiece, but did it also include the Mona Lisa? Charney found much confusion as to this point, so he decided to dig deeper. The result were these two articles, which have some overlap, but both of which examine whether the Nazis stole the Mona Lisa...or just thought they did:

Did the Nazis also steal the Mona Lisa? – CNN
Did the Nazis steal the Mona Lisa? – The Guardian

You might be wondering how artworks are authenticated. The process is labyrinthine and is best illustrated in one, in-depth case study. This long-form article tells of how another suspected Leonardo work, a drawing, was discovered and authenticated:

Is it really a Leonardo? – Aeon