If he had left of his own accord, how far really could he have gone? I haven't looked at the family's home in a while - is it very rural? I recall it being more suburban. Public transport typically has cameras and I assume these would've been chased up, if that was even available near to the home. House CCTV from the area would've been searched, too. And I'm only considering Sebastian leaving on his own WITHOUT a plan - meaning no prior arrangements or meeting somebody online who would 'pick him up'. I guess my mind is still open to three options, each with their own questions.
a) Family is involved: Where and when did they hide Sebastian? What about the evidence, too? Why mention the bump sound upstairs? How have the police not found anything?
b) Sebastian left, without any planning, to run away: How did he get far enough to not be found yet? Which direction would he go to not be seen on any CCTV? Why did he run away? Where would he be running to? What did he bring with him? Is the bump noise related to that? Would he have stopped to sleep somewhere that night, e.g. a forest?
c) Sebastian made arrangements or came into contact with somebody online: Have all of his devices been checked? How would police not trace that? Did he pick Seb up, or live locally?
MOO.