From Halopedia, the Halo wiki
A nanobug is a kind of nanotechnology employed by the Office of Naval Intelligence for surveillance purposes. Nanobugs are tiny pieces of technology that can be used to intercept communications; a nanobug may be placed inside the microphone of a given communication's device and then used to eavesdrop on conversations. Nanobugs only activate when the host device radiates on its frequency, allowing the nanobug's transmissions to be camouflaged as random interference inherent with all radio communications. The technology is so effective, even the Forerunner archeon-class ancilla, Intrepid Eye, was considered unable to detect its presence.
During their infiltration of the Keepers of the One Freedom, Mark-G313 planted a nanobug inside the comm disc owned by Castor, allowing the Ferrets to eavesdrop on his communications. This was later used by them during the Second Ark Conflict. Nanobugs can also be surgically implanted inside a living body, something Veta Lopis used to conceal a nanobug within her jaw with the intent of calling in an orbital strike during the Battle of Epsilon Clarion.
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