A few seasons ago, I found a particular blood trail after dfcleaning the place out that belonged to no dwarf in the fortress I could find, through searching manually and looking at the dwarf list with the "u" command. I've noticed then that this particular character has a habit of leaving stains right after dfclean usually in a trail like manner as if the entity was bleeding. Dakost Admiredboard the blood belongs to, although I'm not sure if that's the full name since it cuts away from it.
Seeing as this was at an ever increasing rate, I decided to look again for the culprit and see what this character was bleeding out from exactly. I checked all the adult dwarfs, child dwarfs, baby dwarfs, and even the pets. Nothing. This was when I hypothesized that Dakost died sometime during my most resent search for him/her, seeing as I had baths set up everywhere there was no way that Dakost could have left these trails of blood all over the place without being alive. I checked the coffins, again nothing. Slabs, nothing, I checked the characters I could engrave onto slabs, and after an exhaustive search came up with similar results.
It is quite clear that this Dakost has never existed. Not only as a part of my dwarves, but he wasn't even part of the besiegers as well. Yet, for some reason, this entity without a body and cannot be found physically is leaving blood trails all over the fortress.
Ladies and gentlemen, I give you the first real haunting in DF.