From Halopedia, the Halo wiki
A microtail was a type of tracking device used by the United Nations Space Command. To use it, short-radius M-waves were pinged to the tracker, which would then reveal its position to the searcher.
In 2544, a microtail was put on Catherine Halsey so the Office of Naval Intelligence could know where she was at all times. This was presumably for her own safety, rather than for malicious reasons.