OK. So due to an unrelated issue with my previous save and Tantares' latest update, I decided to start a-new.
First launch, launching a satellite into geo-sync orbit, everything's fine in the VAB...
And then this happens... I call him Jeff.
Anyway, the serious part:
Apparently, the game has decided to spawn a weird "ghost" asset of one of my subassemblies (specifically of a Centaur-derivative Upper Stage), used in the featured craft. It's "inactive" - i.e not part of the actual vessel and not a vessel itself, but partially interactable, as in I can highlight its' constituent parts. For some reason, the RCS thrusters at the lower part of the "ghost" follower appear active, though it doesn't actually move.
Jeff likes to tag along for whatever you're doing, but keeps his distance. He's also persistent between game reloads and active vessels...