A Telemetry Probe is a UNSC (most likely developed by ONI) device. The only known usage of these devices was to attach it 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 it to the destroyer UNSC Iroquois was based upon this technology, and then the United Nations Space Command discovered the Spy Probe and reverse engineered it. [2]