Smell a rat，可不是闻到老鼠的意思，而是
感觉事情不对头，感觉有点不对劲
怀疑有问题，发现不对头，有所警觉
感到不妙，感到有可疑之处英文释义：
Oxford：
INFORMAL
begin to suspect trickery or deception.
开始怀疑有欺骗或欺骗行为。
例句：when he died, investigators were called in and soon smelt a rat
他死后，调查人员被叫来调查，很快就发现了可疑之处
cambridge dictionary：
to recognize that something is not as it appears to be or that something dishonest is happening
意识到某事并不像表面上看起来那样，或有不诚实的事情正在发生
例句：He's been working late with her every night this week - I smell a rat!
他这个星期每天晚上都和她一起工作到很晚，我觉得有点可疑。
collinsdictionary：
If you smell a rat, you begin to suspect or realize that something is wrong in aparticular situation, for example that someone is trying to deceive you or harm you.
例句：If I don't send a picture, he will smell a rat.
如果我不发照片，他就会觉得不对劲。
smell a rat这个短语可以追溯到16世纪，意思是怀疑某件事在某个特定的情况下不太对，相信某人在试图欺骗你，或者环境并不像他们看起来那样。这个短语最早的用法不仅和老鼠有关，还和猫有关。因为猫是控制老鼠数量的一种最古老的方法。这个短语的常用表达方式是I smell a rat, Do you smell a rat?
内容参考：
https://grammarist.com/idiom/smell-a-rat/
