Sybille Stentor the court Mage in Solitude doesn't show up on Detect Life but shows up on Detect Undead. I've looked her up on the UESP and there seems to be no quest involving her as being an undead. She does send you to clear out a Vampire dungeon but that is about it.

Is there any special reason why she is classed as undead?

There was a mage that I bumped into at the Blue Palace that mentioned something about Sybille having people tortured and killed. When I asked him for more details, he said he gave away too much and should take his own advice (watching what you say). I don't know if that is another quest line, since I never saw him again (he was hanging out in the lobby).

I've seen him walking around a few times. Maybe she's a vampire?

Well if you talk to her you find out that shes much older then she looks so she probably is a vamp.

Seems like it's remnants of a scrapped quest.