1
general::
Happen to, befall, be the fate of, as in I haven't seen foe in a year; what has become of his book? The King James Bible has this idiom (Genesis 37:20): "We shall see what will become of his dreams." [Late 1500s]
American Heritage Idioms
2
general::
Phrase(s): become of someone or something
to happen or occur to someone or something. • Whatever became of Joe and his friends? • I don’t know what became of my other plaid sock.
McGrawhill's American Idioms And Phrasal Verbs