The Piano Room is colored differently from the rest of the mansion, like how the Tatami Room is, and is generally colored with a white scheme. In the middle of the room there is a grand piano. On the grand piano is a splat of some red liquid that can be wiped off with the handkerchief after putting detergent/soap on it, revealing a three numbered code for the safe.
The Piano Room is actually a great room for wasting time in while being chased, assuming the player is skilled enough, as one can loop safely around the grand piano while still not letting the Oni turn around, and waste however much time it takes.
Another thing to be noted is that in at least Version 6.23, after you wipe off the grand piano with the handkerchief, the Oni will be behind you, so be prepared for a chase scene.