So we have been speculating that B and S bought drugs on Monday night @ Billie's moms house. Maybe from her, maybe from her connection.
But that has always made wonder, like you, if Billie knew that Hailey was missing at that point.
I don't think so. I think Billie shared in many of Shawn's hobbies, but I don't believe she knew just how extreme he had taken it. And if the child *advertiser censored* goes back a few years,it may indicate that Shawn chose Billie BECAUSE of young Hailey. He always knew where he was going with this sick fantasy, but he pulled Billie into the edges of it to make it even more extreme.
I think Shawn snuck into Hailey's room late Sunday night. Messed around with her, drugged her. Then he rushed home Monday morning and waited for her to wake up, to see if she remembered and was angry. She did and she was, so he took her away from the house, and came home to tie up loose ends when DD and friend arrived home.
So when he went to pick up Billie, who called him earlier than usual for a ride home, he said let's go score some chit, because he needed to keep her preoccupied to give him another day to clear up details. [ he had help from friends imo]