Before the recent terrain update and before the changes to ground detail I had a slow computer and had to have ground detail set to the minimum. That made all the trees and ground objects not very far from me invisible. I could see an enemy tank dot from miles out and start to range it. Even though I could see the dot many times my shots would seem to explode in mid-air. They were hitting the, invisible to me, trees. I suspect this hasn't changed so, even if you can't see them, they are still there.
As to hiding behind a windmill, tree or other object, if any part of your tank is visible to the enemy they can get a shot on you even if you can't see them. I've killed a lot of guys in that situation and been killed that way.