Yup, a dusty sensor. BE CAREFUL. If you see dust on it (of any size), do not wipe or blow on it. Buy a squeeze bulb blower (no compressed air). If the particles are stuck, you can buy a sensor cleaning kit at any decent camera store. Dust on a sensor is not that rare. On my x5s I pull the lens off every time and check for specks. The smallest ones will even show on your images.