From Halopedia, the Halo wiki
The Department of Colonial Security (DCS) was an intelligence organization of the Colonial Administration Authority dedicated to monitoring the Inner and Outer Colonies. The DCS was incorporated into the Office of Naval Intelligence by Margaret Parangosky sometime after she achieved the rank of Captain and after the CAA "fell from grace" because no other agency wanted it. It had no preexisting loyalties to anyone else in the UNSC, making it extremely useful for her climb up the ONI hierarchy. DCS agents, such as Mike Spenser, were still operating in 2553, monitoring insurrectionist activity on Venezia.