My understanding at the time was they were on heading mode out to the coast of Alaska. They apparently neglected to select Nav and couple heading control to the INS. From Nome on the coast, instead of being in Nav, they retained the last selected heading which fitted with the approach to the Alaskan coast. It was calculated with the winds at the time, this would have fitted their ultimate position at shoot down off Sakhalin. No great mystery, and dare one say, there can hardly be a pilot flying who has not made a similar mistake, if only for a few moments. I recall the 2 minute alert light would still illuminate even though you could be way off track, it would still indicate 2 minutes to passing the abeam and switchover of INS legs.