Had a bear hunter scoping a clearcut that I was archery elk hunting in many many years ago. Before he got to me, I raised my bow and pointed it at him with my arm up like I was drawn back. He jumped back when he got to me and drove off. I use my scope to check out animals once they are spotted, but not to look for game.
If I saw someone scoping me, I would definitely scope them back, fair's fair.