Smell a rat
Definition 1 of 1
Pro Tip 1/3Suspicion Trigger
Use 'smell a rat' when you feel something is off, especially if you can't yet prove it. 
He was late again, and she began to smell a rat.
Pro Tip 2/3Informal Use
It's common in casual conversations, not in formal settings or written documents. 
We all smiled, but privately, I could smell a rat.
Pro Tip 3/3Dishonesty Hint
'Smell a rat' implies you suspect dishonesty or deceit, not just that something's wrong. 
The accounts didn't add up, making the auditors smell a rat.