When Bravo One was downed he ran into the base's hangar looking for Dunbar but he ended up being attacked by Spinners. Dunbar is likely to have died during the Chimeran assault as the only known survivor at Fort Anchorage was J. Robert Oppenheimer. Dunbar could have been infected in the base or died during combat.
Dunbar is never seen and is only mentioned by Bravo One.
Dunbar could have been a Sergeant because Bravo One shouts: "Sergeant? Dunbar?".