are you sure your tests aren't botched? like, do you know about the 'cone of vision' mobs have?
you could walk past a mob that's really close, unstealthed, if they have their back to you.
turn the mob around and make them face you while you try to walk past them(even stealthed) and they'll see you, even from further away.
it's all about that cone of vision
I dont have a stealther but Iv been in Ops, we are standing outside the bosses agro range waiting (always waiting in Ops) and one of our scoundrels comes up BEHIND us stealthed and the boss kills him, its totally bizarre, iv not seen it for ages coz we learn but its a really old "bug"