I had exactly the same thing happen to me Saturday morning.
Invisible thief in EotM killing the entire zerg, with absolutely 0 damage showing in the combat log.
I managed to record this, a long with bringing up the server IP and sent the evidence to Arenanet, but have yet to hear anything back.
Hopefully Arenanet will acknowledge the problem soon, but in the mean time I will be attempting to collect more evidence to keep sending to them.