Wow! Now that's really stupid of them, but from what I've read it's a shock how poor museum security can be.
If I've read this correctly, part of the problem may stem from the nature of the museum's emphasis on science, with the artworks being a side show due to inheritance of the works.
But one way or the other, somebody should have been alert to changes in the artwork.
If I've understood US law correctly, art object proven to be stolen and that are "found" in a purchaser's collection can be forced to be returned. Sounds as though the Europeans have something similar. In which case, someone in the art community - institutional level maybe - ought to contest return to that museum if they're so careless or clueless about its value.
BTW, from what I'm reading worldwide museums are rife with forgeries, even some of the biggest names in the museum world. Shocking!
I expect that when my paintings are eventually hanging in the Met or the D'Orsay, they will be more careful. I'll instruct my heirs to insist on it with them. LOL