If you have cameras in room 111 after 3:33 AM you can see something.
In one of her dialog lines Hana asks if there is a ghost in the building. I wonder if she asks because and somehow doesn't know she and her book go missing at night or if she is just undercover. And the mailman. I bet he is the one behind this all. I bet he spied on the girls and somehow got them into the cult then left the job to be the mailman so he could still be in the building without anyone thinking anything of it. It could explain why he is so against you spying on the girls and why he mentions that there is something that happens during the night.