Who wants the Cerberus automated turret schematics?
I just found the Cerberus automated turret schematics on Tuchanka and got a corresponding entry in my journal that someone on the Citadel might be interested in those.
Who on the Citadel wants those turret schematics?
Solution 1:
You have to wait until after Priority: Citadel 2. The "someone on the Citadel" you're referring to is a C-Sec officer who will appear in the Presidium Commons after the previously mentioned event.
Solution 2:
It's a C-SEC officer in the Presidium commons on the Citadel. He should be on the level overlooking the shops.