When someone does something sneaky or stupid, then when they are caught say "I thought you knew." aka "I didn't think you needed to know"
Your girlfriend cheats on you with some guy, then weeks later says "I thought you knew"
When you are given reasons that justify you cheating on someone. Reasons could include: someone thinking they are smooth, short-term memory, or just stupidity.
"Yeah, you fucking wonder why i've been cheating on you." "Keep being a fucking idiot... you wonder why."