Derek hadn't known at first that there was someone there. But since Chris was breathing, had a heartbeat he could hear, and the wind shifted enough to bring him his scent Derek knew pretty quickly. Of course because of who it was he didn't bother to turn around from what he was doing since he was stacking up wood in a little pyramid to dry out completely for fires.
"You know you don't have to just sit there and watch Chris." More often than not he was without a shirt here but Derek had his on at the moment. He was kind of glad about this since it was awkward enough to see Argent on sex island.