I have a feeling that something must have happened with the trash. The timing would make sense as to why they couldn't find him after the trash was picked up Monday morning (the morning he went missing.)
In the Chronicles of Olivia interview, Katie said that on Sunday after dinner Sebastian took the trash out (it was his chore) and he took the trash can to the street.
Now I am listening to another new interview on youtube and Chris mentions that he used to make Sebastian clean his room, and if it wasn't clean enough, he would come through with a trash bag and bag up everything on the floor that wasn't supposed to be on the floor. Chris says in the interview: "It goes in the trash bag. Tie it up. I've done this to him twice. One time mom talked me out of it. The other time, Sebastian here's the bag, go put it in the trash can. And then you're going to take the trash can to the street."
44:35 is when the section on cleaning the room starts:
Is it too far fetched to think that, Sebastian could've gone back outside to look in the trash for a toy or something of his? This trash can has something to do with it IMO.
It could also explain why he went out in only his socks or barefoot (no shoes) if he intended to come right back in.