Why did they hide in Gen 3:8? Surely they knew they could not hide from God.

Perhaps they thought they could. Again, we have many indications that they had been quite childlike. “Knowledge of good and evil” did not mean they understood all, of course. This was just the first of many fruitless attempts to hide from the face of God. It was also the first of many reiterations that no one can, in fact, hide from the face of God, who knows all of our sins.