Players footsteps don't make enough noise (any noise?). I've often come face to face with another player when turning a corner and having no idea that they were there, and neither did they. Even sneaking should make at least a little noise at close quarters.
Also, on a number of occasions I happened to see someone nearby while in a building. I hid in the shadows and waited until they were gone -- or tried to. I couldn't hear anything to indicate anyone was walking around. Meanwhile, I could hear them opening a soda and eating nearby.