Visible crew/equipment: When Vincent and Sam are running through the ice rink's changing rooms to get the case off Gregor, the camera passes a mirror. If you pay attention, you can quite clearly see the boom operator wearing a blue and white checked shirt in it. (01:43:40)
Visible crew/equipment: During the initial chase with the Audi S8 immediately after the messed-up arms deal, you can see exhaust fumes and tire smoke from the leading camera car in most front shots of the gang.
Visible crew/equipment: In the mountain chase scene where Robert De Niro pops up out of the sunroof with a bazooka, a close shot of the Mercedes' right front tyre reveals a shadow of equipment ~ possibly the camera.
Visible crew/equipment: After they find the case, De Niro throws the case under the car before it blows up. Then the French guy drives a car up to pick him up and get away. Look at the window when the door is open, you can clearly see the reflection of the boom mic.
Visible crew/equipment: Shortly after the AudiS8 chases the CitroenXM through the fish market, both cars then go into a narrow street. There is a rear shot of the Audi going into the street where you can see camera equipment on the roof of the car. In the next outside shot of the car it has gone.
Visible crew/equipment: During the scene at the whiteboard where Robert De Niro is asking Sean Bean about the colour of the boathouse at Hereford, the shadow of the boom mic can be seen on Robert's forehead.
Answer: There's no way of knowing what was in the case. It is a plot device called a "MacGuffin," a term coined by director Alfred Hitchcock. It doesn't actually matter what the object is but is just something that drives the story. It can be any type of object or device such as a "secret formula," "enemy war plans," a "nuclear weapon," a "treasure map," and so on that the characters are either searching for or protecting. There was never any intention to reveal what it was. Its purpose is to motivate the characters' actions and tell the story. Most likely it was intended to be a McGuffin just to keep the audience guessing and thinking about the movie long after it ends.
raywest ★