That was a piece of non black background in the game scene, needed a clean dark area of scene so the figures were clear
Game is Still Wakes the Deep, but the same would apply to Hellblade 2 too as well (UE 5.4) which is similarly well optimised.
Thanks was just wondering what game was tested as you didn't state, next time maybe full screen screenshot so can see what it was rendering at the time too with the data so can see how intensive the scene was as you could look up at the sky or a wall and would read silly high FPS too.