found a solution here, guys: the problem seems to be generated by Download Expander!
To prevent the quantity box from displaying make the following edit to your include/search.php file:
Find this code near line 966:
$products[$k]['appearance'] = func_get_appearance_data($products[$k]);
BEFORE insert:
// Download Expander :: www.alteredcart.com
if ($active_modules['Download_Expander']){
require_once $xcart_dir."/modules/Download_Expander/download_functions.php";
$products[$k]['distribution'] = dlex_is_esd_assigned($v['productid'] , func_get_default_options($v['productid'], 1, @$user_account['membershipid']));
}
// End Download Expander