I am almost certain I read this story on the Sam and Dean Slash Archive when I first started reading Supernatural fan fiction. It's pre-series and I think Sam is maybe 12-ish. John goes on a hunt and leaves them in a rented cottage in the woods and Sam comes to Dean with stories from the local paper that turn into a hunt. I know that sounds like a million stories, but the two elements that are clear are that one of them sees a guy walking beneath a streetlight who seems totally normal until the realize his feet are backwards (yikes!) and at some point, Sammy gets abducted and stashed a shed and Dean rescues him after killing the evil thing that was going to feed off him or already was feeding off him. IT WAS HELLA SCARY!