The marker shown below indicating the location and the date of 3 September is outside the village of Luzarches, almost directly north of Paris. From maps it appears that Luzarches is just about 22 km from, say, where the A-1 expressway crosses the Seine, or Le Bourget Airport. Which German unit made the probe? I can't find it specified in any of my sources, but I would hazard the guess that it was a cavalry detachment from the IV Reserve Corps of Alexander von Kluck's First Army. IV Corps was guarding his flank as he veered to the east of Paris. Corrective information is welcome.