What was the sign made of?
I had a plastic (high density polyethylene) barrier drop on the roof of my Thunderbird and it looked like it left scratches but it was the combination of me driving, the barrier dropping and the hot sun. It was like the plastic had sintered itself into the roof, but a little buffing (ok, my fingers bled during the effort) removed 90% of it.
Had to keep it hot though, that hurt the most.
