He is becoming so "smooth" and at ease with her. . familiarity, I guess. . but that made the ending scene all the more hard to take. At the end, he couldn't even talk to her, really. I think that was what she picked up on as being a "red flag" that something is wrong.
She was pretty upset when she came back to Buy More and she could easily missed it, the only thing she needed was Chuck and he was the only thing on her mind.
She could have seen it when she is showing Awesome and Ellie the bracelet, she did look at Chuck in the moment.
But she had a lot on her mind and she was not vigilant in that moment, I wonder if she even had seen a "bad guy" in that moment.
Chuck holding Sarah's hand and stroking it after giving her the bracelet was a really nice, touching moment (sniff - OK, I am soft like pudding!)
Chuck did really brake her wall in that episode, especially in that scene
Chuck can really touch her buttons, we saw that many times in this episode.