Your opponent has two Treasure Maps, one regular and one written on a blank. He has one card in his deck, and you know he has the regular Treasure Map in his hand (but not the other one), because he revealed his hand for some reason.
Now he plays a Haven, draws the last card in his deck, and sets aside a card. Then he reshuffles and it's your turn. You play a Pillage. If you see the regular Treasure Map in his hand, you could probably safely make him discard something else, as you can be sure he did not set aside the Treasure Map. But if you see the other blank Treasure Map, you might be inclined to force him to discard that.
There is surely a much simpler and more general way to express this idea though