After a Frost Dragon decimated every npc that was outside in Riverwood (except the kids), A thief spawned inside the Riverwood Trader. He's there every time I go, and he just runs around looking suspicious while everybody just ignores him.. The last time I went there he was actually cooking! When he noticed me he turned around and was like "Damn! he caught me being unthief~like. Do something evil, Quick!" He draws his dagger and puts it away again, then resumes hes running around routine.