Sounds are the most common ways of detecting other players are near, and using a decent headset works wonders. you can hear a player running over ammo or the click of a uav, etc even movement through grass dropping off a step gives them away, Our clan play cod 4 and I often get "how did you know I was there" typed up on the screen, you ran over some ammo and I turned and seen you??? and that's from our own clan members, learn to play and don't just run about shooting, I always stop for a couple of seconds listen then move, plus you get a big red dot on the map that gives a players possion away,and the direction of the shot is a good give away.  tactics use tactics I tell them, don't just run around like a newbie