Yeah in the new Farsight Supplement it was said to be from an ancient Xenos society that actually pre-dated the Imperium, living on the planet Arthas Moloch. Whilst it never explicitly states that the weapon doesn't harbour a daemon inside of it, the fact that Farsight dislikes Chaos (having glimpsed through a portal to the Eye of Terror) is a hint that it is simply a piece of arcane technology. I like to think of the Tau more like candles amongst giant searchlights. You can see them, but the moths will naturally be attracted to the strongest light. And I don't really like the Necrontyr theory. The Galaxy is a massive place and everything doesn't have to be because of the same handfuls of races. I wouldn't mind if the Eldar/Necrons did meddle with the Tau at some stage, but I like that they're their own people - for better or worse.