Genesis 4:9

And the Lord said to Kain, Where is Habel thy brother? And he said, I know not; am I the keeper of my brother?

אי הבל אחיך, “in which place did you bury him?” G’d phrased this as a question only in order to give Kayin a chance to begin the process of repentance, not because He was not perfectly aware where Kayin had buried Hevel. G’d did not want to be forced to kill someone already guilty of the death penalty.

לא ידעתי, “I do not know.” He thought that the question concerned Hevel’s whereabouts. Seeing that Kayin had not been interested previously in G’d’s ability to know the future, or to become close to Him; such knowledge about G’d was reserved to those who are cleaving to Him.

'ויאמר ה, the reason G’d spoke to him now was only in order to show him that he could not hide his deed from G’d, and that whatever man does, or even plans to do is fully known to Him. Kayin had thought that by burying Hevel he could hide his deed from G’d, just as he had been able to hide it from his father. This is why he had killed Hevel in the field where there had not been any witnesses. The additional reason why G’d addressed him was to inform him of his impending punishment. The reason that G’d phrased His remarks as if He questioned him, saying: אי הבל אחיך, was only an opening line, just as He had asked Adam in 3,9 איכה, “where are you?,” knowing full well where Adam was hiding. He also had asked Moses in Exodus 4,2 מזה בידך, “what is this in your hand?,” knowing full well that Moses held a staff in his hand. There are more such examples of a question by G’d being for purposes other than to obtain information not at His disposal. Seeing, however, that He had not said to Kayin: “why did you kill your brother?” Kayin thought that G’d was not yet aware of Hevel being dead. He denied all knowledge, saying: לא ידעתי, השומר אחי אנכי, claiming that Hevel had gone about his work just as he, Kayin, had gone about his work.. He added, gratuitously, that he could not be expected to know the whereabouts of his brother all the time, seeing he had not been hired to be his keeper. He acted far worse than his father at the time, as Adam had never denied that he had done wrong. He had only made excuses for himself. Adam was well aware that G’d was well informed about all such matters, and that even though He had commenced the conversation with a question, it was not because He had not known the answer.