If somebody stuck collecting clues, as I was:
There are 6 clues total.
4 clues in the room with the side door, where the key is found. 2 in secret compartments (paper and crystal), 1 paper is the random box or shelf and 1 blue coin.
2 clues in the office, you can investigate it only at midnight. Procurement paper in the drawer and notebook in the safe.
I brute-force the combination, it was 316 (is it intentional or is there a clue somewhere how to open the safe?)
After that you have to wait for several days.
EDIT: Tried replaying for a different ending. First time playing I purposefully let priest fully corrupt MC.
According to dev there is second route where you expose priest by collecting evidence and not reaching 300 lewd before that. Yeah, that's buggy and/or fiddly as hell. This time both secret compartments didn't spawn - so I only collected 4 pieces of evidence. Also reached 300 lewd while I tried to avoid all optional encounters with priest. So, yeah, this is bad.