Scouting and reconaissance are done in a couple of key ways based on what it is you are trying to accomplish.

I'll copy an old post of mine on this subject since it deals with enemy armies locating each other:

The old adage is that battles take place along the lines of communication. This is as logical and simple a concept as you can imagine. It's easier to communicate with the people you need to communicate with, and there's a reason people use that path, well, for communication, so typically speaking you're going to see battles take place around areas that are easy and routinely traveled. Not necessarily on those sites, but around them. Armies can march through rough, rugged, shithole terrain (see Hannibal's march through the swamps to avoid engaging the Roman army, Alexander, B. (1993) How Great Generals Win p.41) but typically speaking this was dangerous, as seen when Hannibal lost a number of his men by risking those very same swamps. So typically, an army wants to use roads, rail, paths, trails, whatever guarantees their destination and ultimately makes life easier to keep a cohesive unit.

So you'd have two armies doing their best to try to navigate terrain where they aren't terribly hard to see, and truth be told, armies leave a lot of mess behind, so properly trained scouts have a decent fix on where the enemy is roughly. Be this by spying an enemy marching down the road with your own eyes, or conferring logical information (a burned up, abandoned camp, discarded shells, tracks from the army, locals giving you information, horse shit, there are a thousand ways) scouts usually had a decent idea of where abouts an army was, if not necessarily where it was going.

Scouts and spies would be swarming around like satellites around an army, some riding on ahead in teams to try to look for clues or anything else that might hint where the army is or where it's going, while "screening" light cavalry would shadow their own army in case something slipped through. Think almost human radar so to speak.

Meanwhile the scouts and the spies would be working in local communities to gather as much information as humanly possible.