I suggested this earlier, but somebody here pointed out that the transponder is set to a particular channel for each flight. So the pilot will always be able to change the station...and I can’t imagine that every frequency is being monitored, and recorded.
But it seems that some variant of a transponder, on a constant channel, could be agreed upon.
The “transponder” is a device that responds to radar. The air traffic controllers assign a number to each flight that the pilot manually dials into the unit. The ground radar system now identifies that number with that aircraft (along with heading, altitude, speed, etc.). The transponder doesn’t send out a signal until it receives a “hit” from ground radar then it sends back a ping with the information.
Someone apparently didn’t just “change the channel,” they simply turned that system off.