يَوْمَئِذٍ تُعْرَ‌ضُونَ لَا تَخْفَىٰ مِنكُمْ خَافِيَةٌ (On that day, you will be brought forward in a way that no secret of yours will remain secret...69:18). In other words, on the Day of Judgment people will be exposed - no concealed act they did will stay concealed. Nothing is concealed from Divine knowledge and sight even today. The Plain of Gathering has been particularly mentioned in this connection probably because the entire ground will be levelled out as a plain surface. There will be no caves nor mountains, no buildings nor houses nor barriers of any trees. These are the things behind which people normally hide in the mortal world. On the Day of Judgment, none of these things will be available and there will be no possibility of hiding.