Sinclair served as a ranger in the Vanguard's Force Recon Unit. At some point after the Battle of New York City, he was identified as gen:LOCK compatible, and was scheduled to join the Experimental Science Unit with the rest of the recruits at the Anvil. However, shortly before he was supposed to leave, he was abducted by the Union and replaced by an imposter. Afterwards, he somehow managed to escape captivity and ended up in Union territory disguised as a Union soldier.
During a patrol, Sinclair breaks away from his group and hides in a nearby alley way. Rounding the corner and taking refuge behind a dumpster, he removes his helmet for a quick breather and, after urging himself to keep going, stands up and walks on, nearly forgetting his helmet in the process.