In a random filler post last month I briefly mentioned my Dromund Kaas stronghold being "haunted" by NPCs that I didn't place in it (and who had disappeared again by the next time I loaded in). It seems that the fabric of reality is getting thinner as we speak, as I'm encountering this issue more and more frequently.
The term "haunting" is really quite apt because the way these unwanted companions pop out of nowhere does have some similarities to a jump scare... except being a lot less scary of course. I think I still yelled when I turned around and was suddenly greeted by these two unusual bouncers in their undies. I don't know if they really appear that suddenly or if I tend to miss them because of invisible gorilla syndrome (which is to say that they may already be there when I load in but I don't immediately notice because I'm not paying attention) but it's startling either way.
My newest Guardian ran into a whole troupe of invaders on Coruscant - Khem Val wasn't deterred by the existing furniture either. What I forgot to take a screenshot of is that she then turned left and found Lord Scourge standing in the corner, taunting her with a quest icon over his head, while she was nowhere far enough in the story to have even met him.
And now, it's even spread to other servers. My Cathar trooper on the Ebon Hawk hasn't unlocked anything but the first part of her Coruscant apartment and I haven't unlocked any companions other than the first couple of trooper ones, yet here are Zenith and Iresso. Why? Who knows.