The third Mass Effect keeper in the Tower is tucked in a circular side area past Rear Admiral Kahoku.The second Mass Effect keeper in the Tower is to the rear of the Council Chamber, to your right hand side as you enter.The first Mass Effect keeper in the Tower is in the open area before the Council Chamber, which Chorban was originally scanning.The eighth Mass Effect keeper in the Presidium is in the upstairs store room at the back of the Emporium.The seventh Mass Effect keeper in the Presidium is between the Consort Chambers and the Emporium, near the stairs.The sixth Mass Effect keeper in the Presidium is near the entrance to the Consort Chambers.Note that this keeper is sometimes bugged and doesn't appear, though you should be able to fix this with a quick save and reload. The fifth Mass Effect keeper in the Presidium is in the Volus & Elcor Office in the Embassies, next to Udina's Office.The fourth Mass Effect keeper in the Presidium is in the Embassy Lounge area, in the room behind the receptionist with several computer terminals.The third Mass Effect keeper in the Presidium is on the balcony at the back of the Embassy Lounge.The second Mass Effect keeper in the Presidium is near the elevator to the Wards.The first Mass Effect keeper in the Presidium is near the elevator to the Citadel Tower, next to an Avina terminal.You'll receive credits and XP for every keeper scanned, with a bonus awarded once you've registered all of them. There are 21 Mass Effect keepers in total around the Citadel, and you need to scan each of them individually to complete your collection.