Ok. Apparently the griefing discovered by Lucas on the map is new. When I saw it I remember that I've been there a few weeks ago fixing griefing damage to Critics buildings there.
After some testing I discovered that the antibuild wasn't working anymore. A permissions syntax method I was using wasn't functioning correctly. After changing to an alternative syntax it now should work again. Fixing the damage was no problem as there are more then enough backups that are very easy to apply trough world edit.
So, for quite some time griefers were able to spawn TNT. While the plugin didn't hinder them building it trough the wrong configuration it still logged TNT placement. So I grepd trough the logs ...
For anyone interested:
Names on this list that placed > 100 TNT on the Creative "worldC04" while protection was inactive:
150 Kreydisian (@Kreydis ) Looking at the coordinates it looks like legitimate use for a building project though.