I wonder if SS returned to the house to "accidentally" factory reset his phone and delete messages off Maddie's.
Remember that JS said she even looked through Maddie's deleted messages -- didn't see anything strange. (While Maddie's friend said she texted SS A LOT -- that IS strange, he co-opted her whole life.) JS also said she said "what happened?" -- maybe because that's what SS told her to say, but in regard to -- "F÷%* what did I just do?" (the deliberate factory reset)
Granted, he could have done that when he returned later but I think it was before JS left for her appt. SS hoped she'd still be asleep IMO. Probably had to go into JS's room to get Maddie's phone, JS woke up, so SS made up the story about walking the dog.
JMO