I have a piece of scrap 2x4 lumber that I was planning to use until I saw these little tunnel markings. Any idea what insect/creature made them? And is there any way to tell if it's still infested? And is it safe to actually use this 2x4?
From the similarity of the lines, I'd say you pulled something out of the old 2x4 (nails) and dragged them across the board, perhaps as you twisted and pulled to separate/remote the board from wherever it was before. I don't believe there are any insects that travel in perfect unison like that.