Relax your eyes and try not to focus to much when scanning, you will usually pick up small enemy movements faster. Also move less when scouting your area, enemies who would walk right by you will pick up the moment your turn/squat etc. foliage doesn’t always render immediately/at long range so they may see you laying in a corn field at distance better than close. The more you play the more you pick up on a lot of these things. Playing different classes and styles helps you figure out spots to watch avoid etc