Hmm. I don't think the rules explicitly cover that case, but here's how I would handle it.
If the undead creature retains the original personality and abilities, like a ghost does, I'd have you find the undead. For most intents and purposes, the ghost actually is the person; he's just not wearing his body at the moment. (If you were actually looking for the corpse, I'd let you find that instead.)
When a person has been turned into a "generic" undead, like a spectre or a wraith, it's not really the character you knew anymore. In that case, scrying for the person would find an image of the body. However, if you know or suspect that he has become undead, you could scry for the undead creature separately. I'd give that a Scry DC no lower than 15, but you could get a +10 on your check if you used the corpse as a focus.