Ok, strange one here, was trying to get to all the Staging points on 31RDM, was running low on MP so picked a nice quiet space hit sneak and invis and healed.
Next this I know one of these is bounding up the hill towards me, I check and sneak and invis still strong, suddenly "you feel a strane presance" pops up in the chat box and Boom! both sneak and invis drop with no warning and 1 hit later I'm eating dirt.
I had 100% HP and 65% MP at the time, WTF!!!