Al knew that Priscilla has been preoccupied all this while as she tried to get out of the castle and she didn't notice many of the explosions and mana surges that were traveling across the kingdom! Her mind has only been on getting to safety first. All of those explosions came from the same general direction and Al couldn't help but think that that was Leviathan trying to fight against someone else. It is possible that Leviathan already came to help them, and he ended up getting killed by someone stronger than him!
But Al couldn't tell Priscilla that! It would only make her more unbearable to stay around and he didn't know what sort of thing she would do to lash out. So instead, he told Priscilla that he was sure Leviathan was just trying to find the perfect opportunity to come and help them. Maybe Leviathan was moving through the ranks quietly so that he can find the demon lord and attack him directly!
