The chest x-ray shows an incidental finding
of Azygous lobe. The azygous fissure is bordering the lobe with a small white density at the lower end of the fissure which represents the azygous vein. Azygous lobe is seen in 1% of population and is a normal variant.
Good to be aware of this incidental finding on the chest x-ray so that the accessory lobe is not mistaken for lung pathology.
Further reading https://www.eurorad.org/case/13436