ELT activated within the aircraft.
I once had a passenger ELT activate accidentally.
((helicopter offshore ops) everybody has one plus the 2 on the helicopter, one jettisoned by impact or manually, plus the 2 in the rafts))
We could hear it but one of our helicopters less than 20 NM away could not.
I presume that the 777 fuselage is much "thicker" re radio emissions than a helicopter so perhaps an ELT activated within with no external antenna would not be picked up by SARSAT.