God is not blind nor is He mocked. If He is omniscient and omnipresent, even in Open Theism terms, He sees and knows all.
God does not forget sins that you, the devil, your buddy, etc. sees and remembers. The idea of God 'forgetting' sin is that He choses to not bring it up again based on a substitute for the penalty of sin. In justification, He legally treats us just as if we never sinned, based on the work of the cross that satisfies love and holiness. Sanctification works actual righteousness into our lives, not just forensic or legal justification.
To think that God could see King David's sin, but could not see the sins of NT believers is dispensationalism gone awry undermining explicit revelation of who God is and His ways.