After her identity has been found out by the Robin Hood Society, and none other than by Arthur himself. Belle knows that her movement from now on will be closely watched not by just her brother but also by Arthur Brown.
She is even prepared in case one of them tries to abduct her and keep her locked in a room in an attempt to stop her from fulfilling her 8th and last kill. Knowing her brother, he would definitely resort to abducting her just to keep her.
During the time that Belle was staking out Pierro Durante's location, she accidentally found Matias De Rossi's as well. It seems that the latter didn't even bother that his cousins were being killed one by one by an assassin and made his presence visible to her.
