3 things come to mind:
- You're getting hit by an AOE
- You're fighting Rikti drones
- It's an ambush and some of them are attached to you
I can think of another possibility. We know that the system has 'shared aggro' (not sure if that's the official name) where a scrapper might attack a group and get attacked in return, but some enemies will also attack teammates who have done nothing.
Presumably this system should not 'see through' stealth or invisibility, but maybe it is.
Paragon City Search And Rescue
The Mentor Project
Has anyone else noticed that if a group member strikes a target it causes the Stalker to be "seen" ? We often game with just 2 and on several occations my stalker who has not struck a target is seen after my teammate strikes his target. Is this supposed to happen. I had never been visible before even if I was standing next to his target.