Upon approaching a locked locker (with a red light) the mod automatically tries to make it unlockable. If successful, the locker will shimmer with a blue light, accompanied by a brief hissing sound, after which the locker's lights will turn from red to green, signifying that it can now be opened.
If an unsuccessful attempt at unlocking a locker happens, the locker's lights will be turned off, meaning you can't unlock it anymore.
The effects of this mod do not stack when multiple players in a party have it equipped. If a locker fails to unlock when Master Thief is applied to it, it will not unlock if other players with the mod equipped also try on the same locker.
It also seems it doesn't stack with Scavenge either, as lockers that don't unlock from the mod will be mostly ignored by Kubrows.