[spoiler]The Wayfarer's are an organisation of guides. They are about the only means to safely (more or less) go through the forests, jungles and rivers, for any group who is not heavily armed. Merchants, travellers and sometimes even heavily armed groups like Keysackran Military or Harren'daen'kar teams rely on them to go from town to town. They do not patrol anything, they are just highly trained rangers and guides, who know some quick ways to get from one point to the other.
A prestige class is the source of their veil. If they were able to spot anything would have to rely most likely on the individual guide. Since most of them are rangers, it is pretty likely.
How they work is like this: You pay them, and they assign a guide to you. That guide will provide you with the Veil and some quick paths. He will not fight for you, nor protect you if anything pierces the Veil (though some have sometimes, again, depends on the individual guide).
If that makes no sense, I will once again insist: THE FORESTS IN THIS WORLD ARE TEAMING WITH MONSTERS OF ALL KIND, and not just the deep parts, it gets dangerous about 50 feet from the city wall. Of course in Keysackra you dealt with this a while ago and don't have any problems with the wildlife. But here, the jungle is a very dangerous place.[/spoiler]