It is a law of physics that for every action there is an equal and opposite reaction. This is true for the physical world and the same truth applies to the spiritual world as well.
There is no such thing as a "secret" sin because of this law. No matter how small we think a sin is we must understand that someday in some way there will be an equal and opposite reaction to it. When that reaction is made known then the sin which caused it will become exposed. So, the old Biblical admonition...."be sure your sins will find you out" is founded upon two things: God's all-knowing nature and the exercise of this law of equal and opposite reaction.
If one wants to know the real serious nature of the "small" little innocuous sin they committed then they should not look at what they consider the "little" sin. It might not have been painful to commit it. It might even have seemed the "thing to do" under the circumstances. But, if they want to know the serious nature of it then they should look at the consequences. Remember our law of physics: for every action there is an equal and opposite reaction. When that reaction is experienced and is serious and consequential then one can get a good idea of how serious the "little" sin really was. Based on our law it will be revealed.
The real lesson is that there is no "little" sin to God. They are all big and serious because of His Holy nature. The consequences will be big even though we thought our "little" sin would do no harm. His Holy nature is at the center of all of this. He is so Holy that even the most innocuous "little" sin is serious business and will elicit a strong response if not forgiven by the Blood of Jesus. How serious was the sin to God? Just take a look at the equal and opposite reaction in order to get a good idea of that "little" sin's magnitude. Like an echo, it keeps reverberating and each reverberation becomes more complicated and entangled as the "little" sin's consequences grow ever larger. Even though forgiven, the consequences must be lived out here in history and the "inconsequential little sin" becomes a major factor in one's life.
William F. Harrell