LE can only request mobile providers to give phone records for victims, witnesses and suspects, so they have to know the owners of the phone to get that info. Phone records for Leah would have shown when the phone was last used and it wasn't reported for the time the witnesses saw. A phone will ping off masts, but depending on the type of mast, the phone can be within a radius of several miles of it. There may not be sufficient masts to triangulate and give and exact position, or a short-range mast that would show Leah within, say, a mile radius of it.