I started noticing this strange patch of dirt on my 1DMkIII sensor, it shows up on all images and can't be removed by auto cleaning or manual dry cleaning. (I haven't tried wet cleaning). It is also very noticeable when I look at the sensor. Oh sure, it can be cloned out but still, I am very disturbed by it and trying to figure out how it happened? Any theories on what is and what may have caused it?