Thief God Milton had just left Pass City for a few hours. He was flying in the air and was searching for a place to land. Meanwhile, he was just notified that a true believer of his died. With the worship passageway in place, it was very easy for him to pay attention to this.
There was another explanation, actually, and that was that he just had too few followers to not pay attention to each and every one of them. If it was any other divine spirits like the Moon Goddess or divine spirits who had a tremendous number of followers, they wouldn't bother giving any personal attention to any followers until something special was to come up. This was exactly the case for Abel. The number of followers he had connected to his druid spirit was about several thousand or more. Deaths happened among them on a daily basis, but he would only consider those as natural phenomena and not something he needed to actively interfere with.