You know when you do something wrong (break a toy and hide it, or sneak a piece of candy) and, even if nobody saw, you get that weird feeling in your tummy? Your heart beats fast, you start worrying?
That's your conscience! It's like God put a flashlight, already turned on, inside your heart. Even if you turn off the bedroom light and hide under the blanket, the "flashlight inside" stays on. It shows you, all by yourself, that what you did was wrong.
That's why the Bible says nobody can hide from themselves. We always know the truth, deep down.