Well, actually, I was once on a flight out of JFK (I think ZRH was the destination, this was 5-6 years ago) and after buttoning up and pushing back, the plane taxied briefly and then had to return to the gate for a maintainence issue. The door was opened, mechanics on and off, crew on and off, and IIRC, some pax were allowed off to make phone calls etc. Total delay was 2+ hours, but my point is that certainly did not have the jetway sealed off to the terminal.