Simon is usually in this particular house of three people. A dad, his wife and a daughter. The dad would be out most of the time because of his job. The mom is a housewife busy doing stuff. So Simon would watch the daughter. She was 15. (Adjust the age, please.)

As usual, she was on her PC doing her stuff. Then she looked behind and widen her eyes. Nobody could see Simon, but it looked like she was staring in his direction. Simon was just staring back, not saying anything until she would speak. He was shocked too, but he needed her to speak for him to confirm it is what he's thinking.