Smell a Rat - English Idiom

This idiom is used to show that you suspect someone of being dishonest or you think that something dishonest is happening. The boss has been working late with his cute new secretary every night. He never worked late before she started at the company. I smell a rat! I don't know what it is, but I smell a rat. Something about this deal just doesn't seem right. When I saw him whispering to the boss, I immediately smelled a rat. They're up to something, I just know it.