Thinking out loud again sorry.
Damn, I'm thinking, there's a report of an escaped convict/mental patient from a near by asylum next to a marsh. The boy shouldn't be out at night in the marshes, but for some reason he is. They're not the main anything in the film, definitely the convict is only in this scene and perhaps you don't see him again.
Dunno, Hound of the Baskervilles?