The hogs also can likely smell the corn at a much larger distance than they can hear a feeder.
There may not be any hogs in the immediate area of the feeder to hear it when it goes off if it has been empty for a while because they have learned that the feeder is empty and so won't have any reason to be nearby at feeding time.
With that said, often hogs are not waiting nearby for the feeder to go off even when it is fully functioning and spitting corn every day, unless the hogs are really hungry (lack of local resources).
I have feeders going off a bit after dark, but may have hogs come in at 2:00 am. That is what, 8 hours after dark and 5 hours before the morning feed time. Such hogs are not in dire need of corn, but are just making the rounds, getting snacks as they go.