I have also seen this when on as a GM when the spell 'weird' is being cast. It appears to be a check for True Seeing (TS) since each creature in the AOE seems to send off that message when hit with the spell.
Log:
[CHAT WINDOW TEXT] [Sun Aug 19 23:02:13] Tegan: TS Spell: 0, TS Effect: 0, TS Listed: 0
[CHAT WINDOW TEXT] [Sun Aug 19 23:02:13] Tegan: TS Spell: 0, TS Effect: 0, TS Listed: 0
[CHAT WINDOW TEXT] [Sun Aug 19 23:02:13] Tegan: TS Spell: 0, TS Effect: 0, TS Listed: 0
[CHAT WINDOW TEXT] [Sun Aug 19 23:02:13] Tegan: TS Spell: 0, TS Effect: 0, TS Listed: 0
.