ㄴ가 떠나다

head off somewhere

떠나다 is leaving or setting off, and the word before it in these lines is a deliberately vague 'somewhere': 어디론가 떠나고 'off to somewhere or other', 어딘가 떠나고 싶고 'wanting to get away somewhere'. The vagueness is the point — the appeal is the leaving, not any particular destination. With 뭔가 it becomes a hedge: 뭔가 떠나는 사람 같잖아요, 'he sort of looks like someone about to leave'.