I've experienced a missing body problem. It was during a goblin siege, but probably not related to it.
People in the meeting hall started freaking out over a dead monster slayer, but only a couple of them, despite the room being crowdy (over 60 people, plus dozens of animals evacuated from pastures). It turned out a monster slayer died suddenly. However, he was still listed as missing, despite death supposedly being seen by some people. I used dfhack to determine the cause of death - it was by a vampiress. I know about her from Dwarf Therapist, but not in-game, so I let her hang around. Plus she's a great crossbowman and mason. Before this kill I had 65 monster slayers, and 55 normal citizens, so loss of a monster slayer is not significant, he left some nice named weapon... and I was in process of furnishing rooms for the slayers and the vampire, so they can be useful in protecting my actual population.
But the death caused some problems. First: how the vampire could have killed the guy without anyone noticing, despite them standing next to the victim? Could it be fog from mist generator concealing everyone, or the vampire is just so skilled? Second: why they know he is dead, stand over him, be freaked and create bad memories, but still not see his body, and remove missing status? With "missing" status, there's no way to memorialise him (he's not on the list), and no one's burying the body.
I have created an archive save for experimenting, and ordered dumping the dead guy into the atom smasher, and smashed him. The first person to carry the body was freaking out, but then gave birth on the way (a nice experience for the baby, too), so another had to continue. He was also horrified, but dumped the body. The dead was still missing! So the guy is horrified because he saw the dead body of the deceased (as it clearly stated in thoughts), and the deceased is still missing and no one knows what happened to him. A bug or feature? (As expected, smashing the body didn't help in anything)
I reverted to original save, and installed a coffin for the deceased, then marked it as his grave. Nothing was happening for a few weeks, except some people present in the meeting hall being horrified from time to time. Finally someone has noticed him drained of blood, and then buried in the coffin, traumatizing onlookers. Then his murder showed up in justice system, I accused some goblin (who died in the meantime on surface killed by an aspiring monster slayer) and let it go.
I know sometimes noticing a body seems to be laggin, but never experienced such a disrepancy between an obvious death and noticing it. Additionally, they can still be horrified by body of the missing person, and not reveal it, which is just stupid. May it be caused by unusual method of dying, mist, low IQ, any rational cause, or is it a bug?