IMO that is as good of a theory as any other. Per stepdad he was able to disable or block chat and seemed pretty confident about that but I'm not a gamer so I don't really understand those systems. I do know that he also played at bio dad's house with what it sounded like a trusted group of bio dad's friends.I'm late joining and unsure if already discussed. He was a huge Minecraft fan, which worries me.
There is a chat on there and grooming is a possibility. If some stranger befriended him over time and earned his trust it's possible he convinced Sebastian to dress in 'night gear' (black clothes/flashlight) to meet up for a top secret mission kinda thing.
A stranger might convince him not to wear shoes to not be tracked or perhaps stranger was initially on foot and walked up to his window to sneak out. I haven't had time to dig deep, has anyone seen pics of LE get foot prints around the window area of house?
That last part actually tugs on my heart a bit. We know from KP/CP interviews SR desperately wanted friends. Maybe he felt really cool to be participating in the minecraft game with others like his few friends at school. IDK