present in the room only in a technical sense
He's aware that they sense he's somehow there only in a very technical sense, that he's got an air of moment's-notice readiness to leave about him.
Context
Hal comes to a public viewing room announcing he's "isolating," and the scene quietly grants the wish: the room fills with people who feel that he is barely there, a body holding a recumbency with the rest of him already halfway out the door. As everyone else dissolves into the film, Hal alone stays unabsorbed, watching the watchers — physically surrounded and functionally absent, the isolation achieved not by locking a door but by a self that never fully arrives anywhere.