At the end of Clone Wars TV show, Yoda is sent to Moraband by Force Priestesses, as part of his trials for learning Force ghost ability. There he encounters ghosts of ancient Sith, and they tell him:
There is no life after death. Only nothingness awaits you, Jedi. Your fear feeds our hunger for power. They will know you are here. We will tell them. You will die and be nothing.
Official explanation is that these ghosts are not real, because Sith truly cannot live after death. Instead, they are just illusions, presumably created by Priestesses to test Yoda.
But later we learn that Palpatine is really alerted to Yoda's presence on Moraband. He and Dooku then try to "break" Yoda and fail. Yet, the question remains: how did Palpatine learn that Yoda is on Moraband if Sith ghosts were not real? I doubt Force Priestesses would cooperate with him (maybe I'm wrong :) )