My buddy and I had completely cleared the Red Fens previously and had never seen them. But, just a few minutes ago, after completing the Claw of Vulkoor mission, a Blessed Drow Scorpion Warrior spawned in the reset Red Fens as I was making my run from the Fire Giants entrance to the Ziggurat and came to my aid in combat. Woo!
I can only confirm one so far, between the Claw and the Ziggurat, as I didn't have time to run the map again. It didn't wander around, but any enemy brought close enough, it attacked.
There were four Blessed Warriors at the end of the Claw mission, so it will be interesting to see if all four spawn in the Red Fens next time I have a chance to run the map after doing the Claw.
I'm pretty sure they will only be there until the map is reset. Will play with it more later and try to get a screen shot.