During my visit last spring to Edinburgh's National Gallery of Scotland, the uppermost floor was only accessible by a fairly tight staircase while an elevator was being installed. I was surprised to see that the stairwell was decorated with empty corbels.
The exhibits both above and below were fairly traditional, so I doubted it was some sort of commentary on the emptiness of art or the like.
Sure enough, a little further on there was a sign that explained not the absence (probably due to construction work) but the work that's normally there, as seen in the photo below. Early in the 1800s, the trustees of the Gallery's predecessor had acquired one of the largest collections anywhere of casts from Roman sculpture, intended as both an exhibit and a teaching tool for young artists.
Known as the Albacini Collection, it still fulfills those roles...when it's at home!