I have the previous 6 series which has a similar sump except for where the wash arm gets its water supply. The soil sensor is in the sump. My previous Miele DW had the sensor in the top right water supply pipe for the upper rack. I don't know if it is optical or what it is since it's not like you see anything when it is operating.
Mine only uses the sensor in the Normal, and Sensor Wash(Auto) cycles. For the normal cycle, there is little to no variation in the cycle time or wash sequence. There is much more variation in the Sensor Wash program which varies the wash and rinse time as well as the spray pressure used.
On mine, the sensing is done near the end of the prewash, and in the main wash, after 9 to 12 minutes. Sensing is done while the DW is spraying and the only way you can tell it is happening is when the spray pressure reduces for about 20 seconds. Following this 20-second sensing, there may or may not be a reduction in the cycle time, and then the spray pressure resumes. In the Sensor wash cycle after sensing if the soil is light, the pulsed spray starts immediately and you get a main wash that lasts about 26 minutes in total.