Well, `Mittel` on its own, is no town name that I am aware of and Mittelpoellnitz was on the march route.
I agree that neither N, nor anyone else in his entourage, imagined that Davout would run into the main body of the Prussian army on 14 Oct. 1806;neither did Davout. They all thought that the main body of the Prussian army was camped on the plateau above Jena.