I have recently re-watched the TNG episode "Conspiracy". To the best of be recollection, I don't think these aliens ever appeared again, despite the fact that "Remmick" was sending them a beacon signal at the end of the episode.
Does anyone remember if these aliens ever reappeared? Has anyone used them for a campaign? I think they might be good ongoing foes, especially if someone the players come to trust turns out to be a host at some point...