Yeah... 8)

The trail depends not on the size of the army, but on the presence of any creatures of level 6-7 in the hero's army; moreover, it changes not just its "clarity," but also various additional features appear, for example, butterflies for druids. That's how I understood it. It's definitely not about logistics or the hero's level. And I've never encountered a hero with only creatures of levels 1-5 (even with legions) and a trail at the same time ;)