Telemetry probe
From Halopedia, the Halo wiki
A Telemetry Probe is an UNSC (Highly probable that it was developed by ONI) device. The only known usage of these devices was to attach to a Covenant warship, and track it through Slipspace, although it could presumably be used for other functions. In 2537, at the Battle of New Harmony, the prowler Razor's Edge managed to attach one to a warship at the battle, and track it to K7-49, a Covenant shipyard facility seventeen light years from the UNSC outer boundary. [1]
Because the Covenant are imitative, not innovative, it can be conjectured that the Covenant Spy Probe used to find the location of Reach by attaching to the destroyer UNSC Iroquois was based upon this technology, once the Covenant discovered the probe and reverse engineered it. [2]
