As memo serves it has already been discussed somehere but where and when .. sorry can't remind myself ..
However not all Bf 109G-10 and G-14 had them painted. Perhaps it could be non standard marking (stencil) added by the maintenance crews or a manufacturer.
Interesting that the location of that circle varies in the above examples. A look through my various references has not produced an answer but at the risk of adding more mud to this, the Buchon recently acquired by the museum I volunteer at has a small air scoop in the area.