I also didn't see anything there, but I didn't left the path, so it's maybe only visible when leaving it. User Iratium tagged it as "Sickerstollen mit Zisterne" https://www.openstreetmap.org/node/7595918896 maybe it's all underground? or maybe spring_box would be better? https://wiki.openstreetmap.org/wiki/DE:Tag:man_made%3Dspring_box