Well i had a short glance on a grunt there, too! I finished everything you need to do and noticed the piano play you are hearing at some points.. and i was just curious and followed the sound "someone must be there, pianos don't play on their own!". When i entered the Window Gallery (the long room with the big windows and high ceiling) i just saw him wandering away from the Piano into the corridor leading to the exit of the Archives. Man i was so scared to go there and leave
but he was gone after that short encounter.
Maybe you disturbed one of his music-sessions, too