So we have two nebulae with barnacles and only one area with UAs (that we know of - Layne/SP and others: the idea of searching Cali for UAs is a worthwhile one for sure).
So, though it's not as if we needed confirmation, the nebulae are key to the Barnacles, but they're not key to the UAs, unless we accept their origin as being in the California sector.
I'm guessing that the nebulae yield these barnacles either because of A) Their makeup (i.e. the types of particles etc that are there) or B) because of their location/origin. If these are the only two nebulae with Barnacles, then the question is what makes them so special. Is it the gamers' principle again? I.e. They're close enough to us for us to find them? Or something materially different about them.
The Pleiades nebula is a wanderer - it didn't form where it is now. What about California?
I strongly suspect that the Witch Head nebula may prove to be another source of Barnacles, both because of its relative proximity to the California and Pleiades nebulae and because it is surrounded by permit-locked systems with a corridor leading in from the opposite direction to Barnard's Loop. If this is the case, then other nebulae towards and beyond Barnard's Loop start looking suspicious and worth investigating - those that are accessible (the Cone Nebula is completely blocked off). I have been feeling close to burn-out looking so I'm taking a couple of days off searching, then I'll head out there again (I've now checked with a known Barnacle site how close I need to be to see them and I think it's unlikely I would have flown past any without noticing, given the altitude I was maintaining).
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As to why a UA shell might only be found around the Pleiades, I keep thinking of 'The Missing'. The Pleiades is hundreds of LY closer to Sol than the California or Witch Head: it would be a bit of a stretch (though maybe not impossible, depending on launch time and the resources available) for a sub-light vessel to have reached them but the Pleiades is well within range of such a ship. Whilst the UA and the Barnacles seem to use similar technology, the UAs have some human characteristics: they transmit
our system names in a human code (Morse) to our ships in deep space, they scan our ships in such a way that our ships recognise and alert us to the fact we are being scanned and they transmit an image, again in Morse, to us, of our own ship, after the scan: this is the kind of behaviour a human might program a probe to do when encountering an unknown vessel of an alien civilisation. Their positioning in a shell and their automatic scanning of passing ships suggests to me that whoever put them there either wants to known what is heading in towards the Pleiades (possibly Merope in particular), as an early warning system, or what is coming out of the Pleiades, as a form of monitoring or intelligence-gathering. A splinter human colony in Merope (or anywhere in the Pleiades) that encountered Barnacles would see it to their advantage to make use of the technology, not only for the extraction and refining of materials but for other purposes as well and might well have constructed the UAs to set up a perimeter to let them know not just when the Barnacle-makers stop by to harvest their 'crops' but also to give them basic information on their vessels and to make a tentative, peaceful, first contact.
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This line of thinking suggests that the Pleiades, and especially Merope, still hold something undiscovered that is related to the Missing. Given the amount of effort that has gone into searching it, I'm not sure what more can be done - it would be terrible to have to repeat our search every time the game gets a new patch, in case a base has been retconned in somewhere people have already been. I think the Unknown Probes may provide us with a clue, when we find them. In the scenario that a human colony repeatedly found itself getting reports from its UAs of passing spaceships, it might deduce from the distribution of these encounters that those ships were coming from the vicinity of Sol. A logical step for it to then take would be to construct a more specialised probe to investigate that region of space to either make a more direct contact or to gather intelligence on possible bases or colonies or to observe activity levels in general. Such a probe might well carry a more direct message intended for the civilisation sending those ships through the UA shell. My simplest guess for the direction the colony would send its probes in would be towards Sol (possibly Achenar) from Merope in a straight line, possibly passing through systems very close to that line. If the Unknown Probes are sent out using the same FTL technology as the UAs (whether they are self-propelled remote probes, one-shot launched from Merope or dropped from a FTL-capable ship) then I would think they would be aiming for the parent star in the systems they are located in, just like the UAs, since this makes sense as a navigational point of reference.
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TL;DR: Witch Head worth checking for Barnacles; Merope may hold something to be discovered relating to 'The Missing'; Unknown Probes may be in signal sources near the parent star in systems on a line between Merope and Sol (possibly Achenar).