i noted on one of the high mountain rescue programmes, the helicopter had trouble locating someone in a valley, they flew to where the signal was coming from in open tussock ground in good visibility and there was clearly nothing there... the pilot decided the signal was bouncing around, he flew to the other side of the large valley and there was the injured hunter there.....
It's not just that the 121.5 frequency can be used by search aircraft; airliners maintain a listening watch on this frequency. So say if you activate a PLB in the Arthurs Pass region, it is likely a CHCH-Australia jet will pick up the signal. At least that is my understanding.
Aircraft also check the 121.5 frequency after landing to check their ELT hasn't been bumped on.
