Sebastian was just looking for items and stuff to sell in his makeshift shop, when he saw an exit sign above a door... He knew it probably ment nothing, but if there was a chance to escape the Blacksite he'd take it in a flash... So he went through the door

and soon he didnt even realize we wasnt tracing his steps... Or uh... Slithering? Doesnt matter, he just didnt know where he was or how to get back to his shop... He was lost for sure...

soon he desides to sit on the floor and try to remember... Or wait untill an Expandable found him, even though that'd be embarrassing for him... So he curls up on the floor in thought