Additionally, one of our admins think it could be related to putting brackets around an entire username. Sounds silly, I know, but when players are on with a name like <subjectnamehere> the ghosts appear.
http://i47.tinypic.com/2yws677.jpg
If you look at the pic, you can see the names with brackets and the times of the ghosts... they are very close. When these people leave, the ghost stays.
UPDATE: Meh... we tested this theory and it is not the problem.