After you speak to Chorban in the Citadel Tower he requests that you scan the Keepers to collect data. There are 21 Keepers total throughout the compound. You will forfeit the chance to do this sidequest if you complete Jahleed's Fears. Here is a list of their locations.

Presidium
Four inside Citadel Tower.
One in the back room of the Emporium, left turn.
One on the balcony of the Embassy lounge.
One in the rectangular room at the right of the C-Sec HQ at the embassy.
One outside Citadel Tower to the left of Avina.
One in the Volus/Elcor office at the Embassy.
One opposite of Avina, halway between the Consort Chambers and the Emporium.
One outside the entrance of the Consort Chambers.
One outside the Wards Access entrance, very close to the arguing Preaching Hana and C-Sec officer.

Wards
One in the hanger bay through C-Sec.
One in the right-most room in the Presidium Wards Access corridor.
One in the alleyway next to the market and Choras Den.
One in the upper market area.
One in the C-Sec Requisition Office.
One in the Traffic Control Room in C-Sec.
One inside Flux, by the quasar machines in the back left.
One to the right of the Med Clinic entrance.
One at the C-Sec Academy entrance.