Upon exiting Shinjitsuki's room when he is dead, the halls leading to the room turn into a bloody purple with see-through floors. There is a hole in the room that
How to Get It
Follow the directions for how to reach Shinjitsuki via the link above and simply use the Nail bat to kill him. Afterwards, exit the room and leave the first hall.
It is thought that Usotsuki perhaps got into some sort of trouble involving whoever Shinjitsuki is in the real world; if Shinjitsuki is indeed the same person as Nanashi, this theory would certainly be more supported. Perhaps she attacked one or both of them, and as a result has been locked up in her room.