itch.io

I'm actually having the same problem. I've done the basement under the kitchen, all 8 rooms of the guest house, the closet south of the library, and all 3 other bedrooms in the west wing. The only areas I can think of that I haven't scoured are the servants' quarters (inaccessible from the outside), the second floor east wing rooms (inaccessible due to giant hole), the room just north of the office (inaccessible from blocked door), and the master bedroom (PC just unwilling to do anything there). 

I'd previously gotten the trophy for killing rats at the start of the game, so I had covered at least most of the accessible areas by that point, so I'm not sure where else I can go now.

At this point, I've resorted to stepping on every floor crack on the first floor just to see if there's a hidden basement full of debris I can fall into.