Third time in a week is significant, that pattern matters more than people realise. Random one-off sightings are easy to dismiss but when something keeps returning to the same location it suggests either a residual haunting playing on a loop or something with actual intent drawing it back.
The end of the hallway detail is interesting to me as well. In my limited experience doing investigations around Lincolnshire, doorways and corridor ends seem to attract activity far more than open rooms do. Something about transitional spaces I think, thresholds between one area and another.
Few questions if you don't mind - does it appear at a consistent time? Does it have any defined shape or is it more of a formless dark mass? And crucially, do you feel anything when you see it, like a temperature drop or just a general sense of unease?
Would really help narrow down what you might be dealing with. Others here will have far more expertise than me but I've been following cases like yours for years and the details always end up being teh important bit.