Collider2D are automatically attached to a Rigidbody2D on the same GameObject as the Collider2D or if none is present then on a Rigidbody2D on the nearest parent GameObject. The property will be null if no Rigidbody2D is attached. In this case, the Collider2D is attached to a hidden body known as the ground body that lives at the world origin as is set to be RigidbodyType2D.Static. No reference to this hidden body is available however.
See Also: Rigidbody2D, RigidbodyType2D.