Sounds like there are at least a couple of loyal Sixt fans here that would rather not believe this incident has happened to me, but the fact is, it did.
And it did happen in Germany, contrary to expectations otherwise.

An e-mail asking for a description of the cleaning fee and what actually needed to be done to the car has as yet gone unanswered. (We just by luck have a couple of pix taken the day of return, showing the car to be "clean", so we'll see what response they bring...)