As shown on the Dashboard, a semi-transparent piece of the fuel tank
@pedro16797 thank you. Its not necessary, almost nothing is and needs to be perfect, but ima let you do your work ;)
This visual artifact is caused by Ambient Occlusion. Crew Cabins use a technique called "depth mask" to show through other parts, and the ambient occlusion uses the depth information to achieve its effect. Both tricks don't play well together. This is obviously a problem, we'll see what can be done to prevent this from being so obvious.
I had this problem too. I think it was caused by SSAO.