You can exclude menus by testing the post id :
add_filter( 'the_title', 'add_cpt_prefix' );
function add_cpt_prefix( $title ) {
global $id, $post;
if ( $id && $post && $post->post_type == 'press' ) {
$title="<span>Press:</span> " . $title;
}
return $title;
}