Lydia hadn't been expecting Derek to come up the stairs behind her and she whirled around, eyes wide as she fell into a defensive stance at the sound of footsteps until she realized it was him. God. Apparently Scott hadn't even made sure that Derek was home before sending her over to pick up the book.
"Scott asked me to come and pick up some book," she said, glancing down at the baby before looking at him again. "I have no idea whose baby it is. It was here when I got here."
And naturally, Lydia hadn't made any effort to pick up the baby or see if there was a note with it. It was left at Derek's door so she figured it was his problem, not hers.