According to Osho, awareness is not another observer behind the mind; it is the very act of watching. When watching is present, it instantly reveals what was previously mechanical and unconscious. Thus the Buddha knew he’d been unaware the first time because, when awareness arose, the contrast became obvious—no logical regress is needed, only the immediacy of witnessing.
You only know you were asleep when you wake up—the light of attention shows the first act was on autopilot.