So I had originally thought the brown part was just the bottom of his shirt underneath his jacket, but I just realized his hands would have to be going through the shirt to be in his pockets the way it appears they are. And it appears to go all the way around his waist unlike a fanny pack. I don't remember seeing anyone suggest this but what if it's a utility tool belt? My dad has a black tactical one that holds his various police accessories (gun, tazer, radio, handcuffs, etc.) and they have a lot of very low-profile ones that can hold a lot of stuff and still sit pretty flat. LE and most Army personnel would have this type of belt for work/service but anyone could buy them online for their own purposes and I'm sure there's a lot of other professions that use them like people in construction, factory work, etc.
I also noticed his hands are at different heights. His left hand is much lower, and though it's hard to tell with the shadow and low resolution, I think his left hand is in his jeans pocket and his right hand is up higher in the utility belt, possibly retrieving a small weapon. Just a theory.
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