you can get back there --Spoilers for the end of the sewers, including a puzzle solution:
Is there any way to get back to the control room on the upper floor, or have I missed that locker now?
And the monitor room is still marked red on my map. Is there a way to clear it? Does the bishop have another use? (it's marked with a ! ) Or is there more than one tape to be viewed?
head back up north and there will be a horizontal staircase that takes you up to a ladder overlooking the cable car, which takes you up to that room -- and the control room only clears as done when you pick up all the plugs, which you don't have to do