I play scout in my group so I constantly look at the map to check for patrols. There are times I'll check a spot for automatons, turn around and back again to see a new pack spawn there behind my back, just standing there menacingly. When you're in a fight and moving around it very easy to walk into a freshly spawned pack.
Clearing outposts reduces the pack spawn rates. There are patrols that spawn from there before patrolling around the map, but I think the game spawns them around the players to reduce tracking random mobs on the map.
We have one or two guys hunting outposts with grenade launchers, so about halfway through a mission we have free reign on the map with little to no patrols and other pack drops.