There was a time when out with a mate, it was getting late about 1 am we were sitting around the fire, it was dry and warm. The moon was just coming over the hill which lit up the entire glen. Sitting there talking cack as you do, when I thought I heard a voice over by the old dirt track. Mate kept on blathering on, so I 'shh'd' him,
"what?" he says.
I reply, "Shhh!"
"What is it?!" he asks again,
"Will you shut it!" eventually he gets the message and goes quiet for about 5 seconds. I explain that I heard a voice maybe two over 'there'. He says I'm talkin bull. After a bit of him listening he goes very still, a sure sign that he's no heard something too. He says he can. So we sit there like scared rabbits listen to a conversation we can only just hear, we can't make it out properly, only bits of words. Various thoughts going through our minds. Who is out there at this time of night. What are they doing? Are they coming here, to our camp. Can they see our fire and if they can't what's wrong with their eyes?
So after what felt like an eternity, my mate realises it's not voices at all, not in the real sense, it was in fact a well known phenomenon. I'll give a hearty slap on the back to anyone who can tell me what it was.
Nag.
P.S. If someone did encroach you camp site, how about something like a mega phone, a recording of a really big dog / wolfy thing and have a few lines tied to trees and bushes about the place.. Play the tape and pull the lines..

I doubt it would actually work of course, but if it did, it would be priceless.