I really like the ghost idea, Diodor. If the ghost followed the same rules as an NPC, the player might be able to count on what he knows about the world and its workings in order to implement some strategy (e.g., player thinks, "Better get moving, this guard will be noticed missing in 10-20 minutes" because he knows the guard''s route)
Another factor to possibly add: How routine is the NPC? If the NPC is always jetting off to Paris or Myth Drannor or whatever, his absence could have another excuse.
I''d love to see something like this implemented for a murder mystery cRPG
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Just waiting for the mothership...