22 Sept 2011
If another player gains control of Master Thief, its control-change effect ends. Regaining control of Master Thief won’t cause you to regain control of the artifact.
22 Sept 2011
If Master Thief leaves the battlefield, you no longer control it, and its control-change effect ends.
22 Sept 2011
If Master Thief ceases to be under your control before its ability resolves, you won’t gain control of the targeted artifact at all.