What does Psalm 139:12 mean?

God can see clearly through the darkness as though it were bright light. Nothing obscures His vision. He sees both sin and suffering distinctly. He notices what we do, whether it's righteous or sinful (Hebrews 4:13). He sees our situations, whether they are happy or desperate (Romans 8:35–39).

In the garden of Eden God saw Adam and Eve's sin as well as the guilty couple themselves. They tried to hide from God's presence, but their attempt was futile (Genesis 3:8–9). When Cain murdered his brother Abel, the Lord told Cain, "The voice of your brother's blood is crying to me from the ground" (Genesis 4:10). When civilization became thoroughly corrupt in the days before the flood, "the LORD saw that the wickedness of man great in the earth" (Genesis 6:5).

But God also sees our suffering. He saw the suffering of the Hebrews in Egypt. In Exodus 3:7 we read the Lord's words: "I have surely seen the affliction of my people who are in Egypt." In his first letter to persecuted, suffering believers the apostle Peter wrote: "The eyes of the Lord are on the righteous, and his ears are open to their prayer" (1 Peter 3:12).