How to create a wordpress template without using any page

You may use template_redirect action:

add_action( 'template_redirect', 'wpse131387_template_redirect' );
function wpse131387_template_redirect( ){
    if ($_SERVER['REQUEST_URI'] == '/some-template') {
        global $wp_query;
        $wp_query->is_404 = false;
        status_header(200);
        include(dirname(__FILE__) . '/some-template.php');
        exit();
    }
}