Jeff Engel studied the feverish crowd hurrying through the subway turnstiles. As he checked each passing face against a card-index mind, he smiled to himself. Even when off duty, the habit persisted. There was always the chance he'd spot a face that would fit, one that would close another active file in Missing Persons Bureau. A mousey little guy slipped through a turnstile and bumped into a fat woman shopper. Engel glanced at the thin apologetic face and then at a briefcase bearing the faded in