Garen made a trip to White Phoenix Forest Academy and couldn't find his cousins, which was when he knew things had gotten bad. It was very likely that they had followed an escaping group and were planning to leave the Kingdom.
He had always tried not to interfere with his cousins' lives, hoping they could live their lives peacefully. He tried his best to leave their lines of sight, and prevented others from finding out his relationship with them as well. After all, his villainous reputation as the Black Fire Palace Master was growing out of hand, it wasn't a good thing to get involved with him.
Although there was no way he could truly fool the significant factions, it was still better than being completely public about it.