I seem to find chests in the Omega Ruins that randomly contain items OR start a battle.
I managed to open two chests once, and got the battle from the third one, and then all the chests disppeared!

What's this?
Do the chests disappear for ever, or do they come back after a while, or do I have to open them correctly on the first try?
Do the chests' contents randomly change every time you reload the file?
Are there miss-able items in there?

Help, please.