one way to tell if it's flux, heat it up and see if it turns to liquid. Not on the fork, of course. It's a bit weird that it's migrating though. And I would think that anything put in there as a frame saver would stay put as well. Flux would migrate if it got wet I suppose, but I'm not sure I see why it would be (essentially) symmetric around the hole.

I don't know how paint reacts to flux, but my experience is that it's okay on steel.