Just a thought: What if the reason we can't find anything in Merope is because the UA's are remnants of a previous event or project. Perhaps the Thargoids were studying us, and their mothership was in Merope when they launched these things, but then had to abort for some reason and left their probes floating around, unable to reprogram them. The UA's seem to point at Merope's star itself, not anything else inside the system. Perhaps whatever it was inside the system they were originally transmitting to is no longer there, and the star is the fallback behavior? Unfortunately, if this is the case I'm not sure what our next course of action should be - any high energy wakes left behind by the Thargoids (assuming thier ships even work that way) would have long since dissipated.
Or maybe its not the Thargoids at all. Historically, the Pleadies Cluster was the location of at least one known INRA base.
Edit: I'm going to follow this tangent a bit here... So, Mic Turner went to the Pleadies looking for Thargoids, presumably because of what he found in Polaris (A Thargoid Shuttle). Instead of Thargoids, he found an INRA satellite which blew him up. Could it have been a trap? The Thargoids in Polaris were the real deal. Maybe the INRA intercepted the Thargoids in Pleione (this is, after all, what the INRA are good at), got them out of the picture, and left the satellite there to deal with Turner. Assuming nobody ever found out, this would be a pretty effective way of keeping the Thargoids bottled up for a few more decades.
Dammit, everything keeps pointing me to Polaris. But I can't go there!