This is the unknown image caught in Casemate 1 at Fort Mifflin in Pennsylvania. Casemate 1 is the location where the bang was heard in the hallway and the benches would come off of their legs during the night.  This is also the location of the fireplace where, according to the story told by the caretaker, the image of a soldier was captured standing behind one visitor.

Unknown image captured in Casemate #1