Given your description it doesn't seem to be something that's getting recorded. I'm wondering if you have Peaking Red on but it wouldn't necessarily just be at the top of the frame unless there was something about the edge of the shot that kept it there. Peaking is a form of focus assist and red is one of the possible peaking colors. It would change depending on the contours and the focus/edge of the object.
Is it a straight line or is it following the edge of an object?
Is there any way you can show a screen shot of it?
Since it doesn't seem to be recorded in you'd probably have to take a shot with digital camera.