User blog comment:FairyRaven/Fairy Tail Chapter 391: Gray vs. Silver, Review and Commentary/@comment-24277053-20140704112330/@comment-7418318-20140704112957

If anyone would show up without being alerted by Makarov it'd probably be Blue Pegasus. In the X791 sub-arc they hinted at Christina having magical sensors with which they kept a watch over the Tenrou-Island area from afar. Maybe those sensors registered something when Cube transformed into Plutogrim and mowed down half of Magnolia or the explosion that destroyed the guild.