Yancey
The ELT unit itself contains the G switch to set it off ....
AFAIK the ELT is mounted outside the pressure bulkhead (usually) and someone would have to go pull the ELT battery out (there's one in the ELT container so it keeps transmitting when it doesn't have ships power after activation), as well as disable ships power. Now you're involving maintenance activities or a large gang of people to go around pulling power and batteries from devices. I know of no evidence pointing to the likelihood of such an organized event taking place here.
And as other non-US airlines have ELTs installed, it's probably safe to say it comes from the factory pre-installed, so one would have been here.
OK - we'll have to disagree
I have fair reason to believe that some 777s (and know from other ac types I fly / have flown) only have non ship powered ELTs, that are manually activated by crew. They are fixed / stowed in the cabin towards the rear. No 'g' switch.
In each case, I also believe / know the more modern deliveries / variants have ELTs mounted in the roof, that are ship's power / systems connected, and deploy by 'g' switch.
But I am asking of a 777 current pilot to confirm (or not) what I am saying, and how that relates to this 777.
NoD
FAA AD extract posts 787 issue @ LHR:
AD Requirements
This AD requires either removing the Honeywell fixed ELT, or inspecting the ELT (for discrepancies associated with the ELT, ELT battery, and associated wiring), and doing corrective action if necessary, in accordance with a method approved by the FAA.
The applicability of this AD is limited to in-service airplanes, which have been delivered with Honeywell fixed ELTs having part number 1152682–2. Future production airplanes will be addressed prior to delivery.
We recognize that various civil aviation authorities (CAA) have different operational requirements regarding the use of ELTs. While the United States does not require a fixed ELT to be installed for operation, operation of an airplane without an ELT in a particular country’s airspace may require coordination with that country’s CAA.
i.e. some regulators do require "fixed" ELTs, some do not.