smell a rat字面意思“闻老鼠”，引申为感觉不妙，发现可疑之处，怀疑有问题，发现不对头，有所警觉。

【例句】
I don't want to say he steals them, but I really smell a rat.
虽然我不想说他偷我的书，但我确实觉得不对劲。

He's been working late with her every night this week - I smell a rat!
他这周每天晚上总是和她一起工作到很晚—我觉得很可疑！