A building near the db sanctuary with an underground bandit base.the base has a treasure room with an unbelievable amount of loot . Check it out. To access it you must press a secret button in the buildings basemant.

http://www.uesp.net/maps/srmap/srmap.shtml?centeron=Pinewatch

How did you even manage to think that Pinewatch was close to the Dark Brotherhood sanctuary

They are on the opposite sides of the town, not really that close in a way.

Sorry , I just remember finding it right before I entered the sanctuary