sound 和safe 从意思上来说是表示回到家时We的状态,从语法结构上来说,是补充说明主语的状态的,所以说,这个是主语补足语.如,Mary came to school very upset.这句中的very upset 就是补充说明主语Mary的心理状态的.
像你说的第一种情况也是主补啊,只不过句型结构不同而已,如,
She was found reading in the library. (reading in the library主语补足语)
主动句：We found her reading in the library. (reading in the library宾语补足语)