In earlier TES I remember enemies popping out of nowhere (not using Invisibility) and crouching to invoke Sneak...but in Skyrim I can't seem to encounter any enemies that ever use Sneaking. But I know they can, since followers crouch when you crouch, and their Sneak skill does matter (why Lydia alerts everyone and their aunt for a Stealthy character as her Sneak never rises, even if you use the enable/disable trick, from 15 skill)

Has anyone encountered an enemy that actually Sneaks?

I haven't seen any actually sneak, but there are plenty who will try to snipe you from a distance or ambush you if you run straight into them.