They're called by many names lurkers and watchers introverts and beholders. I think though, one name suits them best, A spectator.
On the side lines, ever silent, so attentive while letting their mind wander. The onlooker of the onlooker wonders what they ponder, they ask themselves in their silent interrogation, “What's the meaning of it all?” He searches himself for hidden meaning, the unspoken truth.
All during his watching he scours the globe braving the dark and lonely pits of existentialism and soaring aloft among the fluffy clouds of optimism.
In time the spectator's gaze is averted, his subject ever distant. He's fallen into the inescapable pit his mind provides. A rabbit hole. A spiral of thought. He is lost, but does not wish to be found.
Not a another soul is in the room now. The spectator's subject is gone, but he doesn't notice. Lost in thought, with a seeming glaze over his eyes, spectating something new, something strange, something scary, something beautiful…
himself, human.