That blue eyeball-type icon thing over mobs heads means they can see through stealth.
I await the flurry of “well, DUH!” comments, but I only just found out. Gaining a few more levels and having some talent points to play with, I recently respecced to get Improved Sap, allowing me to bonk humanoids over the noggin then tiptoe off giggling without being detected. To get this, I also spent five points on Master of Deception, making me harder to detect in stealth. At least, that was the theory.
I’d seen the blue icon over the head of certain mobs (I think it was added in 2.03), but wasn’t sure what it was; checking with others in a party they couldn’t see it, so I presumed it was some mysterious addon I had that they didn’t. Without sap, I’d wait for the tank to pull and ambush mobs as they ran by, so stealth hadn’t been much of an issue, but puffed up with my new crowd controlling role I boldy strode (well, silently snuck) ahead of the party. “Don’t you worry your pretty little heads, I’ll take care of these nasty orc chaps!” Bonk! And off we went.
The second group, it started to go a bit wrong. Sneaking ahead, performing a couple of insulting emotes towards my intended victim, his dog gave a bit of a growl… and sank its fangs into my leg. Just a moment! I’m the Master of Deception! The Sultan of Subterfuge! I’m a Camden Leisure Pirate! How could this be? I probably don’t need to cover in over-much detail the failings of the “Lightly Armoured Rogue Being Set Upon By A Couple Of Dogs, Their Handlers, Some Of Their Friends, And Demon Who Happened To Be Wandering By” technique of pulling… but somehow we just about survived. And the following group, my effortless sapping once again worked. The group after that… dog bites rogue. It was a bit vexing; I was starting to conclude that instead of “Master of Deception”, I’d spent five talent points on “Blow Kazoo And Wave Banner Saying ‘I Am Here’ While In Stealth”. The fact that it was always blue-iconed mobs that saw me hadn’t quite clicked; fortunately, the feral druid creeping around in cat form noticed the correlation, and it finally dawned that (i) you only saw the blue floating icon while in stealth, and (ii) maybe it wasn’t such a good idea wandering up to those mobs and waving at them. It all went much more smoothly from there.