121.5
It is monitored from every airport and also from low orbit satellites....and many planes in the air.
A couple of things in the story strike me.
The flight was single engine over water. I would hive filed IFR. Even in a twin.
Had the pilot filed, IFR there would not have been the Chinese fire drill to find the aircraft.
I quit fling over 20 years ago due to age, but there was this thing about the air defense zone (ADIZ) which required aircraft to file. And this aircraft also would be equipped with a transponder set to a specific code unless operating VFR.
And not following the rules was frowned upon. One could end up with a very intimidating guy in a fighter off your wingtip.
But I never flew that route so maybe ethe rules have changed.