It is indeed from bandit forts not dens.
From the wiki
This quest can only be found in bandit forts, you have a 50% chance to have the note spawn in the bandit fort, it looks like a green question mark placed in a sub-room event.
To increase your chances for bandit fort tasks appearing it is advised to run Fighter Guilds tasks to increase the task quantity they offer on the notice board.
Whichever way you recruit her the quest line will start after 3 days.