Poirot had opened the door, and meeting her agonized eyes had nodded gently. 
Poirot realised that she was in agony the instant he saw her face after opening the door.
mnli hypothesis: Poirot realised that she was in agony the instant he saw her face after opening the door. premise: Poirot had opened the door, and meeting her agonized eyes had nodded gently. 