From patch notes 4.1.5 - Update 19:
A visual effect will now appear over your character’s head when using Detection Potions, Magelight, or Expert Hunter to notify other players of your ability to see sneaking/invisible enemies.
I have spent several hours in Cyrodiil since the patch and the only Big Red Eye I have seen is the one over my own head. I am sure other players around me have been using Magelight and expert hunter because I can see them making the gesture to cast those abilities.
It doesn't seem to be working as described.
Given that the introduction of this visual effect was accompanied by a change to Magelight and Expert Hunter which make them break the person casting them out of stealth, I really have to ask what use is any of this to anybody?
PC EU