It doesn't matter how publicly available the rules of this FC were, they do not amount to the law of the land or terms of service, they are a social construct within that FC only.
The contents of a FC chest are *NOT* public property, they are company property that is made available for individual use by authorized company members. There's no victim shaming in pointing these things out, nor is there any reason to return to the OPs flawed question. It's very frustrating when someone abuses their company chest privileges, you're certainly in your rights to report that player for griefing and disrupting your play, as are the other members of your FC. Other than that, there is nothing you can do because the player was in fact authorized to access the chest.
The GMs are not repo men, they are not going to recover the property that you believe was stolen, because it was taken by someone with authority to do so. They might discipline the player for the disruption of other players and griefing, but that's an entirely different matter.
The one thing that would 'help' this type of situation, would be the ability to set a maximum number of items, amount of gil, number of shards, crystals or clusters, that can be withdrawn by rank in the span of a day or week. That would slow down such abuse, but it would still be up to the FC master to set appropriate permissions and limits.