In the very early morning, my husband and I heard a lot of noise at the front door and I went to see if Kate was locked out.

locked 是动词 lock 的过去分词形式，表示"被锁上"的状态。lock 本意是"锁住、锁上"。 out 在这里是副词，表示"在外面"的状态。 locked out 是一个常用短语，表示"被锁在门外"，指因为门被锁上而无法进入的情况。