Thanks Timmy.
The Barnacles are the part of the puzzle that is confusing me too. Unless I was wrong about the UP being reverse engineered (as I have wondered a few times) and it was actually the UA that was reverse engineered.
*replace 'Feds' with whoever you like.
This is what I've always believed, that the Probes were in the game before the Artefacts. The line of thought being that 'something' was using the probes looking for 'something', I believe that we could have found it in game first. However we didn't, as most people tend to ignore non-land-able planets. As a result they just moved the story on, on it's own having the Feds discover one, research it (Sirius Corp, anyone?

) and create the Artefact and deploy it as some sort of sensor net looking for unknown ships. So then players find the Artefacts and find out they are scanning for something and transmitting the data of ships scanned somewhere. Can't remember it the Commander who first found the Probe Convoy USS was actively searching for anything linked to the Artefacts nor not, but regardless eventually the elusive 'free floaters' were found. Then like moths to a flame we start playing with them all over the place. This works with the current appearance of USs. But are these unknown ships from the ones who sent the probes or the ones being searched for?
Of course, If the Artefacts were placed there by Forces Unknown and were found by the Feds and again researched and whatnot, ending up with Probes being sent to various Ammonia worlds looking for Something. Probably in the belief that something from an ammonia world was involved in the Artefacts. Based on the little we know, Thargoids like those so lets start with what we know then move on to other places. Makes sense to me. Players come along and start finding the Artefacts, meaning a lot of scanning by the Artefacts etc so the forces unknown are getting a lot of data. So now we have to ask ourselves was this a tactical mistake on the the part of the Feds, letting a lot of data get transmitted? Or was it a planned tactical manoeuvre, to ends as of yet unknown? And now we have unknown ships appearing... is this in response to all these ship scans? Were they the ones who put the Artefacts in place? Or maybe the Artefact was looking for these guys?
In truth both ideas work well.