Corpus of Latin Ecclesiastical Writers, Vol. LXII: Ambrose's Exposition of Psalm 118

Does he alter merits, he who says: Darkness is round about me, and the walls cover me; will the Most High not be mindful of my sins?

10. Would you not have feared the presence of angels, if you believed them to be present? Would you not fear, I will not say to commit, but to speak or even to think what is wicked, if divine scripture had persuaded you that God is the arbiter of thoughts, the truthful witness of secrets, just as He Himself says: Be my witnesses, and I am a witness, says the Lord God, and my servant whom I have chosen? You fear a man who is present; do you not fear the presence of God the Father and the Son? But you do not want to believe, lest you be able to take heed; you do not want to hear, when it is read, that God knows the hidden things of men, lest you begin to know what you should fear, and fear lest you sin. Hear, therefore, divine scripture, that you may turn from a wicked and malignant way. Do not, like a blind man with bodily eyes or like a deaf man, who, because he cannot see or hear those present, believes himself to be alone, and in a crowd of many, while he thinks no one is present, sets out to perpetrate what he believes to be secret—for he who does not see cannot see those who see—similarly, do not you, blinded in the eyes of your mind, estimate that you commit a crime without a witness, because you were able to avoid the presence of a man; there are more who may convict you than those you were able to avoid. You cannot flee from yourself, your own accuser, whom your own conscience brings before you; and if you deny it to others, you do not deny it to yourself, and if you lie to a man, you confess it to God, and if you wish to deny it, your own thoughts convict you.

11. Angels stood by Elisha whom he saw, and therefore [he saw] the armies...

References: Ecclesiasticus 23:18-26 (referring to the sinner thinking he is hidden); Isaiah 43:10; Psalm 43:22; Jeremiah 33:3; 2 Kings 6:15-17 (the story of Elisha and the heavenly host).