Are you on (even partially) an Evil Biome?
Your presumption - that body parts require a necromancer to begin to walk about - is false. The evil biomes in DF2012 (.34.X), Terrifying, Haunted, and Sinister, all will routinely raise the bits and pieces of anything dead within their bounds.
Unfortunately, undead in this version have some considerable advantages - they can be killed, but not destroeyd by conventional weapons. You may strike them down, only to have them rise again with full "health" moments later, since the effect of the Evil biomes is regular, not a one-time thing. Even the strongest and stoutest of your dwarves are liable to be worn down by attrition, and since you can't move your fortress off of the evil biome, you have no choice but to constantly fight against the living dead.
So to answer your question, there might not even be a necromancer.