Strange Object in the Sky

A few days ago I mentioned that I have a big project in front of me to scan my early photos (35mm) and negatives, spanning from 1980s to early 2000s. Here is one photo I captured near the Toolangi State Forest (Victoria, Australia) during a controlled fire, in the early 2000s. At the time I didn't notice the strange object hovering over the mountain. It was only when I had the photo processed that this object was in the image. By the way, I had the negative examined at the time by 2 photo laboratories (Melbourne & Sydney). There were no scratches etc on the negative.