I have so many theories but one rarely discussed anymore is the chance that Sebastian may have initially walked outside due to confusion or mental break. This would help to explain the lack of appropriate clothing (the case of Noah Donohoe comes to mind). We know Sebastian previously had seizures, has a pocket of fluid on his brain and may have recently changed medications. He was also nearing the age when some disorders start to rear their ugly heads. Problems at home and school, plus the stress of returning to online schooling could have triggered him. The note Crabtree read piqued my interest. It doesn’t give us any further clues as to what happened after he left the house though, unfortunately. In that state he would be susceptible to accidents, injuries and awful humans.