不声不响------as quiet as a mouse

例句:
A: Did she take French leave at the party last Saturday?
她在上星期六的宴会中不告而别了吗？
B: Yes, it's impolite of her. She was as quiet as a mouse and left.
是的，她真没礼貌。她不声不响就离开了。

同义词：
as silent as a mouse
as still as a mouse