Elena wasn’t the only one that noticed the fire and activity around the Blood Assassins headquarters. Paveluc was the first person to receive the report that Carlisle’s army had managed to sneak into Lunen and launch an attack in the middle of the night, and the Grand Duke was able to reasonably infer that their goal was to save Elena. There would be no reason to attack the Blood Assassin headquarters otherwise. If anyone wanted to inflict damage on Paveluc before the war, it would be most effective to attack the castle or a military location, not the Blood Assassins.