If you move into location and then discover that it doesn't provide good cover, you can use the undo button, and then take a different position.
> And once I had a moment, when my merc didn't see enemy behind the fence, but enemy saw him.
That can happen. The fence could be blocking his line of sight, but still the top of his head is visible to the enemy. There are some aiming visibility problems in the game. In a future version I am going to add more ray tests to alleviate the problem.