It would appear that some things aren't identified at all. I've just found someone listed as a "female dwarf" with 2194 non-notable kills, who "tamed the giant cave swallows in the depths of the world", fled from 59 different settlements, was a former member of 53 civilisations (and an enemy of one and a general of another). I'm thinking vampire, but I'm not sure. Can't check either, she got herself shot two years before the end of world gen.
EDIT: Nope, not a vampire. Or maybe it is. I've got one of those identified further down the list. The whole thing is similar, except that they're identified as a vampire and there's nothing about going into the depths of the world. Perhaps a warebeast?
There is also another dwarf further down the list who made a journey into the tundra, tamed some polar bears, then started getting attacked by various people until she died of old age. It certainly means something, but I'm not sure what.
EDIT2: I've got a goblin with more or less the same story, except that they only managed to kill one person (notable this time, the first one) before being murdered (and it was described as a "murder"). The only constant seems to be some kind of violence.
EDIT3: Well, scrap that idea. I've now got an elf that made three journeys into the wastes of various kinds, tamed camels and alligators, then promptly did absolutely nothing for the rest of her life. Some dwarf has gone for one-up-manship and made six trips to the depths of the world, tamed stuff on all of them, then immediately died of old age. It appears to be necessary but not sufficient for "raising general suspicion after a murder".
EDIT4: It's certainly not the only source of warebeasts. I've found a profaning one "[GOD] cursed [VICTIM] to assume the form of an [ANIMAL]-like monster every full moon in [LOCATION]". Confirmed, however, that warebeasts aren't identified in their titles in legends mode. Nonetheless, the above is probably not a warebeast, as they seem to get "devoured xxx in xxx" rather than "raised general suspicion after a murder in ...". Vampires do have the general suspicion message though. Perhaps these are vampires that are created later, and that causes the trips into the wastes, rather than the wastes causing the murder sprees?