I thought you knew
When someone pulls a stupid or sneaky move and then acts like a idiot when caught, they say 'I thought you knew' like they're the smartest person ever.
You stole my lunch and then said 'I thought you knew' when I caught you.
He cheated on the test and then said 'I thought you knew' when the teacher called him out.
She texted her crush behind your back and said 'I thought you knew' when you found out.