Here is the full list of locations for Citadel Keepers for Chorban's scanning side quest in Mass Effect.
Players have the option to pick up this quest in the Citadel Tower directly following their first hearing with the Council regarding Saren. Following the meeting adjournment, upon departing down the stairs, Chief Ashley Williams will make a comment about a suspicious-looking salarian over near one of the Keepers. Indeed, there is one, standing only a few feet before them and kneeling down near the creature's side.
When approached, the salarian—Chorban—will attempt to play off his activities. It's up to the player whether Shepard presses them and consequentially offers to help him scan the Keepers.
Otherwise, Shepard can find him in the lower Wards Markets after speaking with Jahleed.
Mass Effect: Keeper Citadel Locations
There are 21 Keepers to scan in total and they are spread all throughout the station. Players will know a Keeper based on their insectoid, multi-legged appearance and pale-green coloring. According to Chorban, the Keepers have been around and taking care of the Citadel for centuries yet no one has ever been able to learn about them. Collecting samples from them or otherwise incapacitating them only causes them to shut down.
This is what makes Shepard's work so important.
Citadel Tower (4)
- Standing next to Chorban
- Far-left of the stairs, when facing them beyond Rear Admiral Kahoku
- Right wing of the top level, when facing the wall, follow the catwalk all the way left toward the back
- Far left corner when facing the elevator before descending to the roundabout
- From the Tower, directly forward next to Avina
- Directly right of the Wards Access corridor
- Up the stairs behind the Emporium on the left-hand side
- Back left of the area to the left of the Emporium
- The left corner just before entering the Consort's offices
- Through the far door of the Embassy Lounge bar, follow the corridor right
- Left-wing of the Embassies, enter the first door forward and find the back of the room to the right
- Right-wing of the Embassies, when leaving Udina's office, turn right into the elcor and volus offices
- In the Wards Access corridor, close to the Upper Wards area
- Inside Flux on the upper level near the quasar gambling machines, far left corner
- In the alley connecting Chora's Den and the Wards Access
- Upper Markets, in the corner to the right of the Expat volus merchant
- Upper Wards, beyond the Med Clinic, take a sharp left at the rapid transport terminal
- In the small corridor connecting the Upper Wards to C-Sec
- In the Requisition Office, directly left after entering the room
- Past the C-Sec Offices into Traffic Control against the wall to the far right
- Far-right corner of the Docking Bay
Upon scanning them all, the assignment will automatically complete and award Shepard their credits and experience.