I don't think I'm missing anything. You want to be able to claim the briefcase as proceeds of a crime, without actually prosecuting a crime. That is unreasonable. If, as you say, "Bob's actions are not in themselves criminal", you've got no justification to take his stuff. You may think it's proceeds of a crime, and you can treat it as evidence of a possible crime. But if you haven't got enough evidence to convict Bob, then you also don't have enough evidence to keep his stuff. Pretending otherwise is a mockery of justice.