The ELT (emergency locator transponder) not being detected suggests that:
1- Either the plane had crashed in such a remote location (I guess Indian Ocean?) that its signal is out of range.
The ELT is dual VHF (121.5mhz) and satelite encoding 406mhz. The VHF signal might be out of range in the open ocean but the 406mhz signal will be received by satellite from anywhere on the surface of the planet.