nvm

It is mentioned in the guild guide:

 

Masterminds

 

Just within the entrance of the guild, Guildmaster Darren Lightfinger and Chief Thief Robin await new recruits. They can be spoken to for information concerning the guild. They also play important roles in the Buyers and Cellars Quest and the capers which follow it to unlock guild features.

 

The "Mark 1" dummy found in the same room can be pickpocketed freely for experience by very low-level thieves. Higher-level thieves are prevented from training on the dummy, although by demonstrating their Thieving prowess to the Guildmaster after a few successful pickpocketing attempts, players can earn a one-time reward of 2000 experience.

 

And one could argue that as it is post-quest it is a feature of the guild and not of the quest and as such does not need mentioning in the quest guide.

Confused as I am, I'll assume this submission is withdrawn.

Locking topic.