Genesis 16 is where this characteristic of God is first described. Guess who discovers it...Hagar! The slave of Sarai who slept with Abraham (after Sarai's command) so that he could have a child. After she did this Sarai "treated her harshly," so Hagar fled. When she fled, the angel of the LORD found her. He told her to return to her mistress and submit to her authority (this is a lesson in itself! God has given authority and we are to submit to it unless it brings harm to His name). He then told her that God would greatly multiply her descendants. She was to name her child "Ishmael, because the LORD has given heed to your affliction." (If you look in verses 11-12 you see that this is where the conflict between Israel and Palestine originates.) Then she declares that God is a "God who sees".
You see, Hagar's problem came after she obeyed her mistress. She was scorned by Sarai after her obedience. It appeared that no one cared for her. But she learned that God sees what is going on and is taking care of her, even in the times when it may seem that He is not there. In fact, He is there and sees ALL, nothing is hidden from Him.
"Where can I go from Thy Spirit? Or where can I flee from Thy presence? If I ascend to the heaven Thour art there; if I make my bed in Sheol, behold, Thou art there. If I take the wings of teh dawn, if I dwell in th eremotest part of the sea, even there Thy hand will lead me, and Thy right hand will lay hold of me. If I say, 'Surely the darkness will overwhelm me, the light around me will be night,' even the darkness is not dark to Thee, and the night is as bright as the day. Darkness and light are alike to Thee." Psalm 139:7-12
So, God has seen. If sin was committed against you, He knows. If I committed a sin, He knows. Thank the LORD, there is forgiveness. However, for those who do not receive the Lamb who takes away sins, there is judgement. This is righteous because God saw it all (2 Thes. 1:5-10). And remember, he sees but He also allows things to happen in HIs sovereignty. We must trust Him in everything and remember that He does not leave those who seek Him (Psalm 9:10). So, run to the One who is sovereign and who sees everything. Trust in His name and rest in it!