this look weird. I have never seen something like this.
Is the buldge empty? (are there something under it that is pushing on the bottom cover)
If no, then that’s more weird. It might be that it’s a small bulge framework deliberately added in to improve airflow. Maybe.
If that spot got bumped or pushed in from the bottom, it could have stretched the metal slats slightly. While you would expect this to result in a bend IN, not out, it’s possible that the slightly stretched material created tension in the slats and due to the specific shape/internal stresses of the surrounding metal, caused it to pop/bow out instead of in once the metal was slightly stretched. Just a hypothesis.
I seriously doubt thermal expansion would cause that. Aluminum is very good at transferring heat, so I can’t imagine a spot that localized becoming super hot unless you were directing a flame at it or something. Plus, once it cooled, it would shrink again. It wouldn’t remain “expanded” after it cooled down.