EDIT: A little background to the question for people saying scouting (and to clarify why I initially posted). This was my first thought as well, but in reading about Roman history, I’d found out that they were notoriously bad at scouting, as it was something that they felt was below them (and it cost them at Lake Trasimene). Thats when I realized I had no idea how these armies would actually find each other
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Terrain and supply lines. Without modern machinery, everything moves by muscle power. It simply isn’t possible to move hundreds or thousands of troops, supplies and arms across steep hills and slopes (don’t be fooled by what a motorized vehicle can carry and climb – muscle power is very limited)
In that sense, it isn’t really possible to disguise where large groups of troops have to march. They follow level terrain where FOOD and WATER is available. (large troops on feet cannot march far in a day, and cannot carry weeks of food/water) Even if horses are available – horses are going to be limited in quantity and most of the support will be carried on carts and backs.
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