Look through the keyhole of the second-floor bathroom door — before you go in.
Official in-game clue: "You don't need a key to see inside. Who is in the bathroom?"
Where: Day 2 Evening, immediately before Helping with Bathing.
Why it gets missed
Timing. The door is a door, and your instruction is to help — so most players open it and get on with the scene. Once you are inside, the keyhole is behind you and the block has moved on.
The clue is one of the few in this game that tells you plainly what to do. You don't need a key to see inside is a literal instruction; the second sentence is only there to tell you it is worth looking.
What to do
- On the second floor, walk up to the bathroom door.
- Interact with the keyhole, not the handle.
- Then open the door and continue.
The rest of the bathing block
After you go in, there are two more things worth catching: the four bathing tiles — Aleksandra, the bath, the suitcase, the severed hand — which the post-credits secret asks for again hours later, and Cleaning the Windows, which wants the window wiped once Alexandra is already in the bath.
Nearby
The full list of hidden achievements, in the order you reach them, is on the triggers page. Going for everything in one run? See 100% completion.