Guy on the roof of the building where the Mayor is a flashing light. Talk to the Mayor get the key, go to the roof top and then open the door , pull a lever kaboom ther you have it.
If I remember correctly it keeps flashing, it's just now disconnected from the other alarms so that the beastlords don't hear it when one sounds an alarm and kill the hostages... I don't even think it shows up on the objective list.
I think it connects all alarms into one. after you turn it off, they can still sound the alarm, but only guards in the nearby vicinity can hear it, not others across the map...