WAIT A MINUTE!

"...they did make a weird buzz/hum sort of noise but it did not seem very loud."

From what little I know of them, they aren't supposed to be loud, their sound is supposed to carry. Lower pitched sounds (like what elephants make for communication) can carry over long distances.

I would suggest you get some helpers and position them away from you (I would go with upwind and downwind, myself) and see if they can hear it. If they can, move them farther away to see what the limit is, even of your crude one.

My questions: If the sound did carry far enough to be viable, how many people would recognize it? And would they recognize it as a distress signal?

In Australia, maybe. Here?

Sue