How Not to Alert Someone to Your Presence

So Isaac was upstairs taking his nap and an hour later he asked to come downstairs. I told him he could come down and went back to watching his sisters cackle at each other while semi-reading my book. I notice out of the corner of my eye that Isaac is standing next to the couch. Then he reaches out and backhands me right in the forehead. I turned and said, "Hey, what did you do that for?" and he said, "I was just letting you know I was there." So I then say, "Next time you can just say, 'Mommy, I'm standing next to the couch.'" And Isaac says, "Oh, sorry I didn't know that. I'll do that next time."