From Protector of the Grey House:
Natalia continued to stare at the wall next to the shower, as her fingers tapped out a nervous tune. There was a space of about five feet long next to the shower stall. Most of the walls had something on or against them: shower stall, toilet, sink, shelves, or mirrors. That spot was oddly empty. She wondered why she had never thought about it or the large empty area of the second floor beyond Vincent’s room. She realized it didn’t matter. She pushed off the tub and went to the wall, as she wondered what the trick was. There was a door here; she would find it.
Natalia thought it out in her head and bounced on her heels as she measured out the possible door. It had to be about this far away from the shower and about this wide for the wiring, if it was an electronic door, and she knew it was. She started to search the wall beside the shower, to find a catch or release that would trigger the door. She took her time, as she knew Vincent would have hidden it well. This was a secret room; the door would be even more of a secret. A tile at about the height of Vincent’s shoulder moved slightly when she pressed against it. She pressed harder and heard a click. The tile slid up into the wall and revealed a keypad.