Well..when I'm wrong...I'm wrong.
Now I must apologize.
For the past several weeks (since we got this CUV)...this line(in first pic) on the screen in the mirror has been centered with our driveway and on screen. When we were not straight with center of driveway..it would disappear(as in second pic)..leading us to believe it was indeed a guideline on the screen. When a sun reflection or glare was mentioned...well I thought...that is silly! However today..the line was off centered...(not centered as usual)...so I pulled out and backed the other direction into driveway. Guess what!...no line in screen image.
So...fourmoremarine was absolutely right! It was a glare each day..on the outside camera. Sounds silly as hell...but look at first pic of the image on screen in rear view. If it was centered in screen(like it has been)and centered with driveway...it did work as a guideline...even disappearing if Equinox wasn't centered with driveway(the angle of glare changed). Comeon...quit laughing...will ya!