Visible crew/equipment: There is a camera operator visible in the reflection of Ethan's sunglasses in the funeral scene.
Visible crew/equipment: Just after Owen Davian is run over by the truck in Shanghai, Ethan chases the rabbit's foot down the street. There is a high angle street scene of Ethan running across the road. A cameraman is visible on the footpath following Ethan. Also, the traffic is going against the road markings. (01:26:35)
Visible crew/equipment: When Zhen arrives at the courtyard in the Vatican, a gentleman opens the car door. A cameraman and crew member are visible reflected in the door. (00:44:35)
Visible crew/equipment: When Ethan and Julia walk next to the river in Shanghai with the Rabbit's Foot, the shadow of the camera moving is visible on the wooden pillar in the left corner. (01:56:38)
Visible crew/equipment: The cameraman covered in black cloth is reflected on the glass of the room in which Ethan is hanged. It happened at least three times in the whole scene. (01:14:50)
Answer: It's never explained. This is what's known in the film industry as a McGuffin, a term popularised by Alfred Hitchcock, which is an item that's crucial to the plot, but the exact nature of which is never revealed because it actually doesn't matter in the slightest.
Tailkinker ★