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Using the Object Inspector on the object of interest to me, it is reported as "Stroked Path: Constant"
The object is a simple red line (indicated by orange arrow above) which overlays several images on each page of the PDF document. It can be selected using Edit Object tool and moved around the page.
The "graphic objects" of which I am referring do not seem to be those created by the Comment annotations as no annotations are listed for the document.