User blog comment:Miskos3/Fairy Tail Episode 257: Tartaros arc: Wings of Despair, Review/@comment-153.91.200.65-20151031151501/@comment-7418318-20151031152033

That was a flashback to explain how she and Mira arrived at the command center to intervene in the confrontation between Erza and Kyouka.

And it was the very point of the flashback to notice her "sneaking" away, although I'd call it openly walking away, rather than sneaking.