I posted this theory in another thread, but I’ll repost here since it’s relevant to the promo for 3×15. Neal somehow escaped Zelena’s curse, which is why he was absent from the story so far. Yet, Neal was unable himself to get to NYC, because he was left behind in the EF. He may have since found a way to travel between realms though (e.g. silver slippers), which is why in the promo for 3×15, Belle and Hook seem astonished when Neal comes bursting through the door of Mr. Gold’s shop. Neal appears from the literal and figurative fog, suggesting he’s just arrived in town. Perhaps he comes to SB in search of his father, only to learn his dad has gone missing. Neal tries to get to his dad first, only to be beaten by the Wicked Witch, who still has the dark one’s dagger. Neal then realizes the only way to beat Zelena is to call in backup, so he travels to Oz in search of Glinda. That could be why he seems “lost” from so many episodes, but like the subordinate male protagonist who disappears before the second reel, he’ll return to save the day in the end.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy