User blog comment:Omega natsu2/Chapter 367: Jellal vs. Oración Seis, Predictions./@comment-190.172.188.65-20140113050538/@comment-7418318-20140113085400

It must be more than just that.

If it was "just" being partially demonic, surely Natsu would have noticed Gray's smell being similar to Lullaby, Deliora, the Galuna Demons, Jackal and Franmalth. While the first three might be different types of Demons, Jackal and Franmalth (as far as we know) are the same kind as Silver, so at least here the similarity should have hit home.