Northwest Airlines
Upon first glance, this logo looks pretty boring, but there are actually two hidden messages in this image. First, Inside the circle, there is both an "N" and a "W" to represent northwest. Additionally, the tiny triangle that creates the top left of the "W" is actually pointing to the northwest, like on a compass. The circle surrounding helps make this distinction...
Continental is a reputable tire-creating company, but you might not be able to tell that just by looking at their logo. However, look a little more intensely and you can see that the capital "C" over the lowercase "o" perfectly makes a tire shape in the negative space. It wasn't until the corporation pointed it out on their site that anyone even noticed!