This piece of skin comes from under second door on the fuselage left side. Just before wings. This part of fuselage, while falling down backside first was burning heavily, fuelled by fuel from Center wing tank, which probably was damaged during rear part breakup process.
The whole underside of the plane is sooted more or less and at least one separated engine fan is sooted too and has heat damage on painting.
The way fuselage is sooted could not have happened during level and stable flight.