In the early morning hours of March 18, 1990, 13 works of art were stolen from the Isabella Stewart Gardner Museum in Boston. Guards admitted two men posing as police officers responding to a disturbance call, and the thieves bound the guards and looted the museum over the next hour. The case is unsolved; no arrests have been made, and no works have been recovered. The stolen works have been valued at hundreds of millions of dollars by the FBI and art dealers. The museum offers a $10 million reward for information leading to the art's recovery, the largest bounty ever offered by a private institution…

…Experts were puzzled by the choice of artwork, as more valuable works were left untouched. As the collection and its layout are intended to be permanent, empty frames remain hanging both in homage to the missing works and as placeholders for their return.

https://en.m.wikipedia.org/wiki/Isabella_Stewart_Gardner_Museum_theft