"_____'s Spirit is Fading Away" Message. What is this?

I just got this message for one of my vampire ghosts! a played ghost, a household at my favorites and I did not reach the maximum sims allowed. I play rotational and many of my households are ghost. My save is not old because I lost my saves with my old PC. My aging is on only for the active household so I do not have ghosts hidden anywhere. I hope this does not mean that played ghost are not save even if they are at My favs list :'(