In Mass Effect, there is an optional assignment called Scan the Keepers that you can undertake that involves scanning all of the Keepers found in the Citadel. Each time you scan a Keeper you earn experience points and credits, so it's well worth doing. Additionally, completing this assignment will add a small reference in Mass Effect 2.
This assignment can be started in one of two ways, speaking to Jaheed and beginning the Jaheed's Fears assignment will guide you to Chorban. Or, after your first meeting with the Citadel Council, one of your companions will point out a Salarian scanning a Keeper and this will prompt the conversation with Chorban instead.
There are 20 Keepers to find in total. All of them are displayed on the maps below with white targets.
Citadel — Presidium (3 Keepers)
Fast travel to the Presidium — Embassies to reach this area.
Citadel — Presidium (2 Keepers)
Fast travel to the Presidium — Wards Access to reach this location.
Citadel — Presidium (3 Keepers)
Fast travel to the Presidium — Financial District or Consort Chambers to reach this area.
Citadel — Tower (3 Keepers)
Fast travel to the Presidium — Citadel Tower to reach this area.
Citadel — Wards Access (1 Keeper)
Fast travel to the Presidium — Wards Access area and then go down the stairs behind you to get to this area.
Citadel — Flux (1 Keeper)
Fast travel to the Wards — Flux to reach this area. Or, take the central stairs going up in the Upper Wards if you haven't been there before.
Citadel — Upper Wards (3 Keepers)
Fast travel to the Wards — Med Clinic or Markets to reach this area.
Citadel — Lower Wards (1 Keeper)
Take the stairs going down on the eastern side of the Upper Wards map to reach this area.
Citadel — Security (2 Keepers)
Fast travel to the Wards — C-Sec to reach this area.
Citadel — Docking Bay (1 Keeper)
Take the central southern elevator in the C-Sec Academy Atrium to reach this area.
Once you have scanned all 20 the assignment will auto-complete. You do not need to track down Chorban to hand it in.