
I thought as much
Confirmation Bias
Use it when something confirms your suspicions or expectations, but don't overuse to avoid seeming like you always expect the worst. 
Not Surprised
Express lack of surprise in a situation where others might reveal the information as though it's a big reveal or new discovery. 
Subtle Agreement
It's a soft way to say 'I already knew' without saying it directly, which can seem confrontational or arrogant. 