The opening of the new podcast S Town begins with the description of clockmakers trying to repair clocks without knowledge of their service history. The clockmaker must look to clues in the material, dings, screw holes, small pieces of evidence to show what work has been done. These are witness marks. Trace physical evidence that signifies past work or how things fit together.
These marks can be intentional — used in surveying to indicate boundaries, or trailblazing to indicate a path. In engineering and construction, witness marks tell a person how pieces are meant to fit together (think the notches in IKEA furniture). Obviously, witness marks are the stuff of criminal forensics.
I like to think of this phrase more metaphorically. What are the witness marks we all have in our lives, be they physical scars on our body or emotional marks that tell of our past. To paraphrase Wallace Stegner, this word is too pretty for just metal.