Though they hadn't officially announced they were dating, Sara had a suspicion that this was exactly what Greg and Sophia were doing. She saw it by the look on Greg's face, every time Sophia came into the room.
Sara knew Greg was dating someone, and for once he had not been telling anyone about his relationship. This was the part that bugged Sara the most, she was so use to Greg telling her everything, or at least telling someone. Greg had simply told both Sara and Nick when they asked that he and his new girlfriend had decided to keep it under wraps until they could determine how serious it was.
So, Sara began to spy on Greg. She told herself it wasn't because she was jealous, she was dating Gil on the sly anyway, but part of her told her maybe it was. She justified the spying by saying, ever since the flash mob case Greg had been so down, and she didn't want the girl to hurt him.
Now, as she watched through the break room window, seeing a genuine smile on Greg's face as he talked to Sophia, Sara felt a pang of sadness. She saw them kiss briefly, and she knew that her suspicions were right. And she wondered if part of her missed Greg flirting with her, and she turned before her mind could stay on that questions. She didn't need to wonder if maybe she should have said yes to Greg on one of the many times he asked her out.