The most important device that is not[i susceptible to any crew action (theories abound) is the ELT. The Emergency Locator Transmitter is installed on [i]all] aircraft here in the U.S., even very small, old airplanes, (like the one I learned to fly in) that have no electrical systems. The ELT is self contained and only activates when a certain, preset "G" level is reached in sudden deceleration.
The ones in small aircraft are about 8 inches long and maybe 4 inches wide and deep. They contain their own battery and can be turned on by the pilot, unlike the 406 MHz emergency beacons installed on commercial aircraft.
The 406MHz variety can also be detected if activated, by certain satellites dedicated to that purpose. Although the satellite detection may not be precise, it would at least the signal.
To date, no such signal has been detected.
I've assisted in investigating a number of aircraft accidents... some of which left the aircraft entirely unrecognizable as such, yet the ELT was still transmitting when the wreckage was found.
Admittedly, scenarios can be imagined where the ELT did not survive impact, such as being at the bottom of a 10,000 foot deep body of water... but in many cases the ELT would survive and transmit.