The UNSC Silberg was a stealth ship of unknown class used by ONI for the Radio Beacon Deployment Program. She was absent from the Battle of Reach while conducting a deep-space mission.
Jason Morelli, a Navy signalman, was a member of the ship's crew[1].
Partial transcript of On Reach.wav
Jersey: Durga is my dad on reach!?... Durga: He wasn't on Reach... Jersey: Thank god. Durga: ...He's on deep space reconnaissance on a ship called the Silberg. Jersey: Whew!