User blog comment:Sky-Dragoon-Twilight/Weisslogia and Skiadrum, Alive or Dead?/@comment-18654001-20141208200051/@comment-9595671-20141209130905

We found another valid theory, Memory Control magic. Grandeeney has been shown to be able to use Telepathy magic, a very common form of magic, so its possible both Weisslogia and Skiadrum both know Memory Control and used it to fake their deaths simply by altering Sting and Rogue's memories.

Memory Control only fades once someone finds out the truth, and since all the Dragons vanished during 7/7/X777 that would make it incredibly hard for Sting and Rogue to discover the truth. Its possible they realized the truth as of Ch 412 even.