Yeah, the shadow re-entered Gold as he gave him the dagger….
Maybe it’s the fact that he didn’t scream in pain? If he stabbed himself, which he did, he would have felt something. Maybe Belle thinks that because he didn’t express any pain that his act of sacrifice protected him and that he actually wasn’t killed, but sent somewhere?
well he did show some pain, I think he restrained himself so not to hurt his family who was watching….