User blog comment:IamJakuhoRaikoben/Chapter 375: Supernatural, Review./@comment-5303512-20140314233709/@comment-7418318-20140315090729

Maybe he just honed in on the strongest magic source he could find, once he got in range. I could imagine that all the FT mages, plus the magic within the island plus Mavis' ghost could together be stronger than the power of Zeref, and the exhausted GH survivors, Hades, Rusty and Kain.

At least the latter three and since Zeref managed to stay hidden for four centuries, he might be able to hide his magic power from sensing attempts at least to some degree.