User blog comment:IamJakuhoRaikoben/Chapter 468: Memory of the Stars, Review./@comment-7418318-20160110092936

Regarding how Natsu and Happy "slipped past" the rest of the guild I just noticed something.

Brandish and Lucy are IN THE INFIRMARY right now (remember that Brandish was choked unconscious by Marin and then treated by Proly). So Happy probably rushed exactly there, since it's the natural place to bring hurt people. With Happy being able to fly and being quite fast when flying, it's only natural that the other guild members would be at least a few steps behind him and will probably burst through the door any moment.