Raina: it IS possible to miss the chest in the CoT (Cloister of Trials) in Bevelle, if you don't have two Bevelle spheres with you when you go to the exit.
As for being able to go back and get it, I can't remember - been too long since I made that mistake. It would make sense that you can go back at some point, but at the same time, Anima is technically a secret aeon, so it could be a one-shot deal.
But personally, I'd place money on being able to go back in at some point later in the game.