before its departure from JFK
Or some time earlier (much earlier ?) in the flight. I assume it was in front of an empty seat although it could have been one occupied by an inexperienced (sic) passenger who didn't think to mention it.
To continue to destination might suggest the crew was not as concerned as the report implies. This was an A320 so a multitude of diversion alternatives were available. Don't know where the fighters were from, but since they 'escorted' the flight to SEA it seems to me the laptop wasn't discovered until they were ~4 hours downroute. Would be interested to know exactly when and how it came to light.