Today has been a really long and productive day in Star Stable. I finally found the final ghost, which is on the Dark Core headquarters barge. Once you've done the initial quest to go out there, you won't be able to return until you are given a quest by Elizabeth Sunbeam, and she won't give you this quest until you reach Popular with the Sun Circle, which I just did today. She'll tell you it's time to figure out what they're up to and tell you to head to Carney at Silverglade Winery to pick up some diesel for Captain Brus. Here's a screenshot of the final ghost on the barge. You have to wait until you're almost done out there, once you've made your way past all of the guards, and you'll receive a quest dialogue to find a vantage point on the helipad as it appears there are people up there. Instead of continuing the quest, I went around the side which slopes upward, and maneuvered around until I got to the ghost, which you can see vague...
Veronika Starstorm's musings and helpful pointers on the world of Star Stable Online