There should be an ELT in the rafts onboard. They are tracked by satellites - but not sure if they need to be manually located or are automatically engaged when hitting water. This is just weird to me.
If the entire plane was hijacked and landed somewhere else, or if it landed in the water or hit a mountain in the middle of the ocean, it should have been located by now. Unless, Airport ‘77 style, it is intact at the bottom of the ocean.
ELT- Emergency Locator Transmittor
When I worked on commercial aircraft long ago, the ELT’s had a salt water (or urine or any conductive liquid) activated battery and transmit at 121.5 MHz and 243 MHz. But if they went to the bottom of the ocean, the salt water would attenuate the signal to zero.
Submarines have special long wire, towed communications arrays that operate in the VLF band to allow communications underwater.