Justin was an ex Warden and knew Bob was on the most dangerous artefacts list (below the Eye of Balor, The Spear, The Shroud, The Crown, The Placard and Excalibur) which in no way should be in the possession of an individual wizard. Ever. So he didn’t tell anyone in the White Council.
He may have told whoever he was working for but the people he was working for weren’t very nice, abducting children, so I doubt he told them about his Ace in the Hole