The fix ELT on the 777 can be set off manually in the flight deck or by impact (G-switch)
If someone deliberately hijacked the aircraft they wouldn't want the ELT to activate, so wouldn't use a flight deck switch, and why have such a switch anyway, if you need the ELT you need the ELT, I guess the only reason would be in the case of an accidental activation - heavy landing ? (!!)
If the aircraft has landed deliberately on some secret airstrip, highly unlikely I guess, then it woudn't activate and the same reason as above would apply.
Maybe ? one could land on the ocean, as per the Hudson ditching, softly enough to not activate it, tho' I doubt it, so I guess the only answer is that it has deliberately not been switched on, or activated on contact with the ocean then promptly sank and was swamped. The water activated ones mentioned already are intended for use in liferafts by survivors, I doubt that they too could continue working X-fathoms down.
So - I don't think lack of an ELT signal is too big a mystery ? i.e. it's drowned, or it's deliberately repressed.