Listening on departure and center frequencies would get him close, and if he studied this for weeks ahead, he could listen to the center and aircraft transmissions and develop a plan.
Of coarse, if you know the transponder was ahead of you you would only need two antennas to establish to vector to the signal.
Or, even simpler still, a directional antenna (yogi) pointed out the windshield of the plane, rotate the antenna with some means to measure signal strength of the transponder signal. Sounds very crude, but it could work. How do you think they find those radio collars/tags they attach to wolves, bears, sharks, etc.