From Halopedia, the Halo wiki
Recon One was a UNSC Marine Corps reconnaissance unit dispatched shortly after the Spirit of Fire's arrival at Installation 00 on March 28, 2559, tasked with aiding Spartan Red Team in locating the source of a UNSC signal. After reaching the surface of the Forerunner installation, Recon One encountered a hastily-created barricade which, unknown to them at the time, was desperately set up by the Ark's scientists. The unit proceeded to set up charges on the obstacle to allow Red Team's Warthog to pass through. Recon One then accompanied the Spartans to a Forerunner teleporter where they were ordered by Captain Cutter to hold position and await Red Team's return. Soon after the Spartans' departure through the teleporter however, Recon One spotted a large force heading to the supersoldiers' position and so proceeded to inform them over COMs.
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