PetaPixel revealed a nice little factoid about the secret symbol that's on almost every camera. If you read the camera's manual, you probably already know what it is but if you didn't, that circle with a line drawn through it marks exactly where the sensor of the camera is located.
In technical terms, it's called the 'film plane mark' and is potentially helpful for people who take macro shots, as knowing where the sensor plane (or film plane) is inside the camera's body let's photographers know the exact distance from their subject. Also helpful for people who love these little fun facts. [Photonotes via PetaPixel]