Scouting a military force's location, movements, and defenses should actually be a simple matter.
Think about it—civilians can enter and exit an army camp, prostitutes, vendors, and military dependants can nest and stay overnight in the military camps engaging in all kinds of trades. Once on foreign territories, each location they reach becomes chaotic as if a "locust army" has invaded a village. How could such military actions be secretive?
Including Charlemagne, armed forces of various nations had undergone varying degrees of modernization, from tactical commands to weapon equipment, compared to the past, it was a complete transformation.
