To find the mysterious briefcase, head to the north part of The Loop, right along the river. You'll listen to a news stand guy yell at a different guy about how important the briefcase is, then the second guy will walk off with it over to the wall.
He'll switch cases here, but keep following him. Eventually he will have walked all over the area, but if you stay up by the Chicago News advertisement, you'll be fine.
The guy with the case will then throw the case into a boat. There is a motorcycle right here on the path, so hop on that and follow the boat along the river.
The boat will make it's way out into the lake and over to a marina.
Go on foot to the other side.
Then jump into the water by the building the boat docks in.
If you are facing the door the boat went through, take the ladder to the left.
Move along the side of the building and just make sure not to get killed.
Once you get to the back of the building, you will be able to open up the briefcase and copy what you need. With that done, simply leave the area! Nice!