Yep......so if you decide to zoom back (or use a chimera wing) to get back to Farebury, there's loads of chests on that first continent that require the thief's key to open.

But before you do that, go to the cat tamer's lodge (I forget the name). Its near Baccarat on the map. Do a little sidequest, and you'll be able to call a sabrecat, which you can ride. This will come in handy finding all those missed chests after you get the thief's key.