After your first visit with the Council, Ashley (or another companion) will take note of a salarian named Chorban bending over a keeper if you move close enough. Speak to him and he will tell you about his work and the grey area of legality it falls into. Eventually, he will give you the scanner and the assignment to scan all of the keepers in the accessible part of the Citadel.All you have to do is approach each keeper and scan it.

Locations:

Citadel Tower

1.Right next to Chorban, it's the one you caught him trying to scan.
2.On the highest tier, near a door in the bottom-left (southwest) corner of the map.
3.Back on the middle tier, in an enclosed circular area to the east, past Rear Admiral Kahoku.
4.On your way out, in a corridor above and to the left of the stairs down to lower tier. This is in the northwest corner of the map.

Presidium
Locational aids for all keepers except the one examined by Chorban. Topmost first 3 images are the Citadel Tower keepers, the next 8 ones are Presidium keepers, and so on.

1.West side, near the elevator to the Citadel Tower, right next to an Avina terminal.
2.Northwest, near the Wards Access.
3.Enter the Embassy Lounge, continue through a door and on to an "external" balcony.
4.Again in the Embassy Lounge, to the northeast in an unnamed room containing computer terminals, human diplomats and an elcor.
5.In the Embassies, the volus and elcor office beside Ambassador Udina's.
6.Southeast, by the entrance to the Consort's Chambers.
7.Beside the stairway south of the Krogan Monument and west of the Consort's Chambers.
8.In the upstairs storeroom of the Emporium.

Wards

1.Upper Wards, behind the Med Clinic, across from the Rapid Transit terminal.
2.Upper Wards, in the north-west corner of the Upper Markets between stalls (left of Expat's stall).
3.Wards, on the northern wall mid-way along the Alleyway between Chora's Den and the Wards access corridor.
4.Upper Wards, Flux, upstairs behind the Quasar machines.
5.Lower Wards, in front of the entrance to C-Sec Academy, near Officer Eddie Lang.
6.C-Sec Academy, in the C-Sec Requisitions office.
7.C-Sec Academy, in the Traffic Control room.
8.Alliance Docking Bay (Dock 422), at the end of the gantry the Normandy is docked with (This keeper will not be present if the assignment Citadel: I Remember Me is active).
9.Ward Access, in a small, north facing room on the corridor between lifts.