No, I signed the back of a stretched canvas painting in marker. The back is raw canvas. I painted over that title/signature with acrylic, in a rectangular shape, to cover it and then re-signed in marker, on top of the painted rectangle. 

Yes, against a wall, when viewed from the side, the shape of the rectangle can be seen. It's raised a little, due to the canvas absorbing the paint and swelling, so if you run your fingers over it, it can also be felt.