I've done this before on another playthrough with no issues.

Sybil says to find a key so you can 'sneak' out of the building into the Alley beyond. Nets you some xp and an item.

She isn't appearing when I leave the building. Why?

I have the quest. I have the key. If I leave the building the quest is still in my quest log.


Edit:

Ack! Nevermind. For some foolish reason, you must use the key on the door. The door which is the most foolish to use, as there is another door in the room to the far south.

Leaving the building without using the key won't trigger the quest. Apologies.