If you do something unobserved, you do it without being seen by other people.
例句
Looking round to make sure he was unobserved, he slipped through the door...他看了看周围，确定没有被人注意，就从那扇门溜走了。John had been sitting, unobserved, in the darkness.约翰一直坐在黑暗中，没有被人察觉。