I noticed something similar in that area two or three years ago. I was on the N1 approaching William Nicol. There was something resembling a rocket moving upwards in the distance. And not so much of an angle as OP pic and had much more of a trail but still looked the same IIRC. My only thought was those amateur rocketry guys (Not sure if any in JHB) but otherwise I had no idea what I was seeing.