From Halopedia, the Halo wiki
TANGO was an Office of Naval Intelligence contingency plan. It was initiated during preparations for Operation: HYPODERMIC, where it was discovered that an AI had breached their defenses and was aware of their plan.
As such, the prowler Circumference had her registration number scrambled, her computer core isolated from the station's computer, and software activated to cease any more AI activity. To the station's computer, the Circumference never existed.