Transponders can be shut off by pilots (happened with 9/11 planes)and enroute radar can only go 200 miles so when a plane is over water, it will not be tracked by ground based enroute RADAR, the track that is on sites like flightaware is a projection of it’s last postion and it’s flight plan.
Only AWACS could track planes over water or a RADAR on a ship could but I doubt either were in service at the time.
If this flight was outside the 200 mile limit or went below the RADAR then it could very well be diverted somewhere else.