In one of the open cells (one at the very end of the hallway), there's what looks like a tombstone. A memorial, perhaps? But upon closer inspection the following things are written on it:

In Memoriam
Mukuro Ikusaba - birthday - the date it's currently on the exploration
Toshirou Hijikata - birthday - the date it's currently on the exploration

And then they're met with a bloody hand trying to grab them from a void portal that appears on top of the tombstone.