How did it get into the library?

1 Deliberately entered via a spell, such as summon library. If the creature is not of the appropriate type/level to cast the spell on its own, someone cast it for them. 25% chance in this case that the passageway is still accessible somewhere on the level.

2 Born here

3-6 Made here (1-2 recently 3-6 in ages long past, now best forgotten)

7-8 Wandered in accidentally through a spontaneous entrance from one of the usual places. (1-3 old section of a mundane library where no one's been recently 4 wizard graveyard 5 magic forest 6 anywhere constructed with or with furniture made of wood pilfered from a magic forest (or stone pilfered from a wizard graveyard)*) 10% chance the passageway is still accessible somewhere on this level.

9 It's always been here

10 The PCs are somehow responsible

*not a recommended building material