Originally Posted by xliioper

Check-in cutoff time is T-30. It's possible the person in that seat never checked-in for the flight and if all the Gold's and higher who hadn't been upgraded boarded prior to T-30 the seat would not have been available at that point. The other possibility is that they were a no-show (possibly connecting from another flight and skipping a segment). These are usually cleared out at around T-15 (although I would think Main 1 boarding would normally begin before T-15).