User blog comment:FairyRaven/Fairy Tail Chapter 357: The Nine Demon Gates, Review and Commentary/@comment-4489126-20131025101054/@comment-7418318-20131025104635

Unfortunately both your points are speculation.

The describtion we got of Last Ages only said it'd take the "time" of the caster, so we don't know whether she also lost her magic.

And even if she lost the ability to use magic, there still might be enough traces of it left within her to track her.

We also don't know just how much the black gremlin thing saw and reported to Zeref and whether Zeref passed that knowledge on to Tartaros.

Then there would also be other methods of divination (like Cana's card magic for example) that might track Ultear down by means other than tracking her magic.

If they want to keep some kind of knowledge that only the council had access to out of circulation, they might still come after Ultear.