Some logic here.
If you consider this from a gameplay mechanic point of view.
First off, the barnacles drop everything that a ship needs to survive for an extended amount of time away from populated space.
Secondly, we have this dispersal thing going on (quite probably) involving the UAs. I'm willing to bet that the UAs are indicators that a nebula contains barnacles, as a UA is much easier to find. Simply hang around (hopefully) 150 LY from a nebulas central star and wait to see if it spawns.
Thirdly, at least one nearby nebula (to the plaiedes) must contain barnacles as they would have had to set up shop there beforehand. (According to the dispersal theory) we find that, rinse and repeat until we trace them back to their origin.
Makes sense to me...time to come back from BL!
This. It's been the basis for nebulae i've been visiting to check, didnt think of the UAs though, will start this and see what i can get!