User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7153535-20130710153323/@comment-7153535-20130711171217

Well, Knightmare17, Nico had been planning to bring Bianca out when he found Hazel the only reason he couldn't bring Bianca back is because she chose to be reborn. So he probably could have brought a person back without Thanatos being captured, he just thought no one would notice one more escaped soul. It may have been possible that he brought back Hazel during TLO and after about a month Thanatos was captured releasing a lot of souls making it possible for Hazel to be over looked. It's unlikely that Nico would have known Thanatos would be captured but he may have heard of something in advance from his dad in the underworld or he may of just gotten "lucky".