I suspect that the reason the UA shell has never expanded out again past its present 135-150LY radius from Merope is because the UAs found what they were looking for, or sufficient signs of it to make it worth deploying UPs to specific locations. Therefore, I suspect free-floating UPs are going to be found within 150LY of Merope. I suspect that the UPs wailing behaviour indicates that their purpose requires being close to a planet, so it might be best to search close to planets in this region.
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The criteria for which planets/regions are of interest to UPs is something that would, as Michael Brookes suggested, only be obvious after the fact and we realise 'oh, that's what they were looking for'. Until then, we can speculate that they might be interested in life of one kind or another or the locations of crashed ships etc. and test that by searching in appropriate locations.
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It is very important to note that Michael Brookes has suggested that it will be easier to find free-floating UPs
when looking in the right place than it is to capture them from the convoys. This means they are almost certainly not as rare as the convoys and may even appear as frequently as UAs do in the shell. That means we can almost certainly rule out Merope 5c as a possible location: it has been searched so much we'd have found them already. We can also more easily rule out other locations, e.g. near Earth-like Worlds, after perhaps a couple of hours rather than days spent searching. This should allow us to refine our search parameters on a shorter timescale.
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The next question is tough: does the image in UP transmissions tell us where to find free-floaters (and thus where they were sent before their capture) or is it a clue to what they were looking for? I suspect we will only find out after we can compare with that of a free-floating UP. I have a hunch we will see another image from free-floaters and need both images to find the next clue.
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Have any 'tip-offs' been reported with locations within 150LY of Merope? If so, these planets might be worth looking near (not in orbital cruise but close enough for the planet's name to show on the HUD).
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What could the UPs be searching for? This would determine the logic of their deployment. Some possibilities to get people thinking:
Earth-like Worlds: (closest that I know of is in the system with the Alliance listening post)
Ammonia worlds
Life-bearing worlds of different kinds
Barnacles (or worlds with an environment suitable for them)
Planets with specific materials (Polonium? Yttrium?)
Inhabited worlds (current human outposts included)
Lost/crashed ships (human or otherwise)
Valid combinations of the above (e.g. Barnacle-friendly with Polonium)
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TL;DR I suspect that we may be able to find free-floating UPs near planets matching certain criteria within 150LY of Merope. By ruling out even one individual world in each category (e.g. Earth-like or Barnacle-friendly), we can hopefully exclude other worlds in those categories. By a process of elimination, this will hopefully lead us to finding free-floating UPs. Images from both captured and free-floating UP transmissions will help us to find the next clue.
