You tape or cover part of the objective lens of the scope as that part is the most easily seen when you are concealed. Think, how many bushes have a perfectly round, black hole/shape somewhere in them? On top of that, a glint of light off of that piece of glass means that you're dead. Snipers that keep their front scope cap down with an uneven hole cut in it are somewhat common.
Now you know, and knowing is half the battle.