lit. to appear and disappear unpredictably like a spirit or a ghost (idiom)
fig. elusive
Definitions (粵典–words.hk)
動詞
好似神同鬼噉出現、消失；形容一個人行動迅速，變幻莫測，成日無啦啦出現，跟住又無啦啦消失，好神秘、好難捉摸 (of a person) to keep on appearing and disappearing mysteriously; to act with great dexterity and preternatural swiftness; literally 'to come and go like a god or a ghost'
Definitions (Wiktionary)
phrase, idiomatic
to appear and disappear unpredictably like a ghost