User blog comment:IamJakuhoRaikoben/Chapter 451: Fairy Heart, Review./@comment-26822192-20150908093320/@comment-7418318-20150908212730

Cube was floating a bit away from Magnolia at the time of the explosion, but already well within visual range (duh'... hard to hide something that big).

At least in the manga it only fully rushed into the town after transforming into Plutogrim. Not sure how they portrayed it in the anime.

And the Cube went to Magnolia on Kyouka's orders so they could deal with Fairy Tail. She commented how they wouldn't have needed to come there if they had known how easily Sayla could take the guild out or somesuch.