How do I catch the hidden threats I still miss?

You read a scene well now. The obvious dangers register before most people even look up. What still gets past you is the thing built to get past you: the ambush, the hidden threat, the one designed to hide. That it slips through is not proof you are failing. It is proof someone worked to fool a trained eye. No one sees everything. The people who think they do are the ones caught worst. Real skill here is not perfect detection; it is staying loose enough to react when the surprise comes, and not freezing when the picture breaks. Your training holds even when your eyes miss. One step: after each surprise, walk it back. What could have hinted at it, even faintly? You are teaching your instincts a new pattern. The hidden threat stays hidden only until you have seen its shape enough times.