I don't know that they sneaked, but that doesn't mean they never came up behind you undetected as you walked down a path minding your own business.
No, they almost always did "sneak". If an enemy doesn't run into your field of view, they rarely ever made enough noise to alert you to their presense before they hit you. Bears, Ogres, etc, all effectively became masters of sneak if you didn't have the battle music cue.