The way I see it, the whole idea of Mullock having a secret vault seems like it would have been because the money there was "dirty" somehow, which would explain why it wasn't simply in some bank somewhere. I mean you could say the banks of Oddworld are probably corrupt, but Mullock was a part of the biggest, darkest "mafia" cartel of sorts so maybe there's some things that should never see the light of day. If Lorne ever decided to really show it in the story (besides that one secret level in Abe's Oddysee) I would imagine there'd be something more important than just money in it, or maybe the money itself was significant somehow besides there just being a lot of it.
There could be some kind of evidence in there somewhere that incriminates him somehow, for something from his past maybe? Gotta remember the whole scrapped lore for him surviving and going to court. Maybe the botched management of his business wasn't the only thing he was being sued for? He was a big player, maybe he had enemies who were just waiting for the chance to have a shot at him?
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