After returning to the Citadel for the first time after finding Liara, completing Feros, or completing Noveria, an Alliance admiral -- Rear Admiral Mikhailovich -- will be standing by the elevator to C-Sec, and he will accost Commander Shepard in passing. However, if the first return to the Citadel is only during the lockdown, the Admiral will not be there, making the assignment impossible to acquire and complete.