Sorry to hear the news Christy
After talking with you today I thought about heat or flow as the cause of your STN. However after reading this thread I can see you have those bases covered and I had a feeling you would have looked at both of those areas.
I had this same problem about a year ago and the more that I think about it the more I think it may be due to what you first suggested-Chemical Warfare!
Last year I had a green milli that slowly died from STN and due to it's slimming over the course of a week or so I 'm pretty sure it was the cause of almost loosing three other SPS located near bye. Try checking in on the reef via flashlight during the night to see if any corals are slimming up or if there are stingers drifting in the water column, maybe this might be the cause of your STN.
Goodluck and keep us posted, Rich