NPC Spawn Issue: Invisible spawn

[Bigcountry23]

I was at the Pained Soul camp and it appeared there was no spawn. Then someone showed up and was talking about me leaving the PH up. I saw nothing. They engaged and started fighting the PH, and then left. I could not get a target off of an assist, could not see anything. As the monster was KOS to me, it attacked me after they gated and my pet was able to kill it, but it does cause a concern of invisible (even with see invis) mobs.

I was able to log out and back again, but had it not been a /list camp, i would have likely lost the camp (not to mention the possibility of blundering into a mob I literally cannot see or target).