If so, then their starship may have been moved. Perhaps it is still on the mountain somewhere. Alternatively, just as they were brought in, they were killed -- and oddly enough, they don't appear to have come back as a usual citizen would. So without them, their starship may be useless, and the overlords here could have simply gotten rid of it.

Have you asked them? I doubt they will give you a reasonable answer, but you can try.