Smell a rat

Father	Why is it pitch black in here ?
Wendy	There's been a blackout, Dad. Why don't you come to the kitchen ?
	(Father walks slowly to the kitchen.)
Everyone	SURPRISE !! Happy Birthday !
Father	Thank you, everyone. This is indeed a surprise but I should have smelt a rat when I saw so many shoes outside.

Explanation :

Smell a rat - to recognize that something is not as it appears to be

Example : He's been working late with her every night this week -- I smell a rat !