Well, considering it's in a briefcase in the woods, I would start investigating the history of DB Cooper. And then I would take it to the police. For a large sum of money like that, I don't think it's totally legal to just take it like that.
But if they had no record of anyone trying to claim it, I guess you're golden. And hopefully there would be a reward if someone did.
Also, your bank balance suddenly spiking up 5 mil? You'd find your accounts frozen and a pending investigation on your case very quickly.