Would it be ok to say 'A rendőr nem lát egy fiút'? Would the meaning be any different?
"A rendőr nem egy fiút lát" -- What the policeman sees is not a boy. (or if you stress the word "egy": The policeman doesn't see one boy (but more))
"A rendőr nem lát egy fiút" -- There's a boy somewhere, but the policeman doesn't see him.
Yeah it's really true and "A rendőr nem lát egy fiút" with a cynical intonation also can be: There is a boy somewhere but the policeman doesn't want to see him.