Obviously at one time it was sealed over by brick which have been removed and it may have contained something. Of note is that the old BFO machines would detect voids like this. Can you tell us more?
It's surely some kind of storage for treasures. It's difficult to imagine what's else it can be. However, I might be wrong, but it seems as a most obvious variant for me.
From around 1700 until the 1940's interior construction of homes had lath and horse hair plaster. It was the material of choice used for finishing interior walls and ceilings. The method consisted of nailing thin, closely spaces strips of wood called “lath” to the wall studs, and then to put coatings of plaster over the substructure to create finished walls. Yours is covering the brick wall.