When you say you ‘Smell a Rat’ you mean that you’re picking up on something
suspicious.
Example of use: “What are you guys up to? I smell a rat.”
No one is certain when or where this phrase originated, however if you have ever smelt a dead rat in a wall, you know that the odor is horrible, indicating that something is definitely out of place. The term probably comes from the days when rats were more common, and when prize terriers were used to sniff them out and kill them.