Heat-Sensitive Paper

Storeroom (second door down the Centaur corridor; first let Jill pick the lock of the first one, the study): on top of the trunk at the back.

Burn it in the dining-hall fireplace and it becomes Password 2.

The storeroom also has 2 Hand Grenades + 2 Incendiary Grenades on a box to the left, a weapon per the item pattern, and Score Star #8 in the corner cage. One door here won't open from this side: you use it during the spike-ceiling trap.