I think it's actually the opposite. In the US, the regulations changed several years ago from "First to Invent" (which is ambiguous) to "First to File" (which is pretty clear). In any event, I think you can only claim something as prior art if it's in the public domain. So, if you're very shady/greedy/lack an ethical backbone, and you get your hands on a private brief that isn't protected intellectual property, technically you can file that patent. I assume the owner could try to prove theft if they can determine how the IP was leaked and stolen intently.