That circular thing in the right corner, it could be one of these two things. The newly found Phaeton gunship that we have learned about has circular engines on its wings. Maybe part of the ship? Chief possibly fought it off and was hurt at the last moment when Locke came in?
The other idea, when Edward and I were playing Halo 4 we noticed how the wings of the Watchers were large, large enough to resemble the wheel of a Warthog.That being said, that could have been the remainder of a Watcher that met his fate fighting Chief or even Locke.