User blog comment:Sky-Dragoon-Twilight/Weisslogia and Skiadrum, Alive or Dead?/@comment-25849146-20141211044435/@comment-9595671-20141211044933

I think by now we can all imply the Dragons were in their slayers, its probably why Igneel told Grandeeney not to interfere and leave. Its possible being in Natsu would have caused Natsu to notice something.

And for how they're alive, the best theory we have that makes sense is they used Memory Control magic like Doranbolt during the Tenrou arc. Grandeeney has shown she can use telepathy which means Dragons are able to learn other forms of magic. (If their normal powers are magic instead of being biological)