。
She popped up out of nowhere.
Literal
She [topic-は] from-where-not-known suddenly came-arriving.
どこからともなく is a set phrase ('from somewhere unidentifiable') for 'out of nowhere.' Built from どこから (from where) + も (even) + なく (without it being). ひょっこり is a 擬態語 (state mimetic) for the manner of an unexpected appearance — popping into view abruptly. やってくる is a compound combining the verbal-aspect やる with motion くる, capturing 'arrive, show up' in a way bare 来る doesn't quite manage.