Wth? how are people doing this? MW2 Question help? (PS3)?

I'm playing Skidrow and everytime I play nomatter where I'm at camping or hiding somebody runs in, stops DEAD and turns straight around and starts shooting at me. I watch their killcam and half the time they don't have a heartbeat sensor or UAV up. I'm either crouched in the corner and before I can even pull the trigger BAM, Stop dead and instantly swirl around and start shooting at me, even if someone is running up infront of them. What the ****? is this like a glitch with some kind of Aimbot or something because alot of people have been doing it and messing up badly. They don't lightly turn around either like they're doing it with their controller it's like they're moving around in some kind of spectator mode when they turn around, not moving feet or anything but they swing instantly around even if something is coming at them or they're busy doing something.