Kids may be pretty sneaky, but most parents are naturally-skilled private investigators. Which is why kids often think they're keeping secrets from their parents, when in reality their parents know everything. They're just pretending not to. It's usually not until our teen years that we learn how to actually keep secrets from our parents. And even then, sometimes mom and dad are still a step ahead of us.
My three year old tells me she washed her hands after going to the bathroom... She can't reach the faucet on her stool.
My 5yo thinks we believe that the "mystery bandit" is really the one forgetting to flush the toilet and eating the last brownie without permission.