As much as I'd love to insert a "wife" joke here, I won't. But, I believe that the plane remained at altitude up to the point that they had realized they overshot their destination. If they were at altitude, they probably didn't reduce their airspeed, which would indicate that they were out of radio contact for a considerable amount of distance before they even approached their destination.
That begs the question, did they not respond to radio calls from control, and did the tower even attempt to contact the aircraft. Long story short, how after 9/11 can a plane either on course, or especially off course, go 1.5 hours in complete radio silence. Why were there no fighters scrambled when a plane has gone silent for well over an hour? It sounds like their could be some other systemic problems at play here in addition to pilot incompetence or negligence.
Notice that no one is calling it a diversion. If they flew over the airport, surely radar picked them up.
Something is up....It's all about "TIME" and we're time freaks in today's world.