I think it's up to the player playing the ghost either as a full character or just as a NPC. I mean, the freedom they have can be limited to the city, the bluilding, a floor or even a single apartment. Having multiple ghosts, however, could lead some to think that the block attracts this kind of presence so they wouldn't necessarily be confined to the building itself? The point is that I don't want to be limiting here. If you want to add a ghost that just passes by for a couple of posts, don't bother with a character sheet. If you want to play a ghost that stays in the building, a character sheet would be good.