I'm wondering why SS would leave his gas tank open? It's just a little thing I know but if my car ran out of gas and I decided to leave it locked and walk home (I'm making an assumption he was walking to his sister) I wouldn't leave the gas cap off for fear someone would mess around and throw dirt or stones in it.
Another simple thing, if you don't have any spare gas with you why even take the cap off to begin with? You can't see much staring down a car's gas nozzle (not like a lawn mower or chainsaw type deal). Was his gas gauge broken maybe? If not, then why undo the gas cap at all if your staring at the gauge needle on E?
Was it so if a cop came by they'd know it wasn't just a dumped car needing to be towed, but a car out of gas with the owner coming back? I don't know that just seems odd since it's such a rural area to be worrying about getting towed anytime soon.
The other thing, why leave your wallet in the car if you're walking home? Was he afraid of getting jumped and robbed, it's safer in the locked car? He had his phone (a new phone according to his Mom and an Apple iPhone according to LE) a device many could also attempt to steal, so why take it and not your wallet?
Maybe someone did come by and offered him a ride to his sister? Would that make him leave his wallet locked up...?