Additional theory here. If it's been posted already my search didn't turn up anything. But I doubt it's new to you gents who have been going at this for a long time. Just wanted to post since it's my current take based on all the work you've all done. Big round of applause to all you Commanders who are working to crack this nut.
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Since the UA's are always present in convoys, we know they are either being picked up at the locations we jump into, or they are in transit.
The intercepted coms chatter states "this mission [...] I asked what this thing was [...] I got told was don't ask". We can imply that they have been tasked to retrieve the UA. So the Feds & Imps would have to had known the regions of space where to look for the UA's, since we have no evidence to say that they knew the actual pin-point locations.
But since they knew what regions of space to look in, someone in the military structure would have had to have known they were there. Unless the UA's were discovered by chance by the Imps/Feds. If that is the case then any theory people have proposed regarding them can be valid. But FD say we're missing the obvious and that it is solvable? So...
If the Feds/Imps military knew what regions to look in, then they knew these devices were there, and tasked pilots to retrieve them. The UA clearly knows its location, given that it is giving off "morse" signals transmitting its general location.
If the Feds/Imps know what to listen for, and are somehow able to track this signal from further distances than we can, then they could pinpoint UA locations and send out retrieval teams. Or the retrieval teams know what signals to listen for when searching for UA's.
If they don't already know the exact locations of the devices, then they have some way of tracking down these devices that we don't have access to at the moment.
Now, this next bit is pure theory:
If the UA's were not an accidental discovery, and their presence was known, and since they are giving off a signal that CMDR's have partially decoded (so information that is intelligible to humans), could these devices be containers for the Mycoid virus?
Then Perhaps they were left in space to deter the Thargoids from returning to our space, or to infect any Thargoids should they return.
And maybe the virus has mutated as is now affecting humans, so teams are being tasked to retrieve them before outbreaks get out of control.