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from the main c-sec area, head up the big (and freakin' slow) elevator that leads to the hangar bay. Once up top, follow the inclined path straight up and you'll quickly come to keeper 21 on the left.

Hope that you got good use from these photos!

from the main c-sec area, head down the steps to the east to the requisitions area. keeper 20 is to the left of the main doorway immediately inside the room.

from the area where you find keeper 18, head down the elevator into c-sec. Once in c-sec, head up the steps to the west and continue up the steps until you cannot go up anymore stairs. to the right of the steps on a lower level, you'll find keeper 19. While you cannot physically access his location, you can easily reach him by approaching the railing.

From the upper market area between the upper markets and the med clinic, head down the stairs leading to c-sec. You'll find keeper 18 near the bottom of the stairs and near the spare rapid transport pods.