From Halopedia, the Halo wiki
USUAL SUSPECTS is the codename adopted by an Office of Naval Intelligence operative, likely a member of Section III, in 2552.
On December 31st, 2552, Codename: SURGEON filed a report to USUAL SUSPECTS entitled "Historical/Psychological Analysis of Cole, Preston J." The report was filed from UNSC Point of No Return in synchronous orbit on the far side of Luna. The language of the report would suggest that USUAL SUSPECTS is SURGEON's superior or superiors.