I'm wondering if the barnacles are found in nebulae because that's a good spot to hide if you need to. All of the dust would be good to help disguise fleets and colonies, and the Thargoids are probably still pretty gunshy about getting too close to humans, after the whole attempted genocide thing. Now, my computer won't run Horizons (not going to ask for a refund, just going to buy a new system in a few months, I need an upgrade anyways ), so I can't come help the search, but I'd think that the places to check would be near Ammonia worlds or gas giants with ammonia life. After all, arn't Thargoids supposedly a form of ammonia based life? Those kind of planets would be good for them, and I'd suppose that if the barnacles are related, they could be mining settlements and/or colonies forming on planets nearby.