Zack is on his way to Cam's office when he sees her rounding the corner towards the exit. Her head is down, so she doesn't notice the two lab technicians who wave at her as they cross paths through the exterior lab doors.
"That's odd. She called me to her office, but she's leaving the lab." Zack says to no one in particular and turns to go back to his office. As he's walking he sees Dr. Brennan sitting in her office. She is staring at the computer screen, but seems to be looking through it, rather than at it.
He sees that she has a pendant between her thumb and forefinger and is rubbing it absentmindedly. The smile on her face is one of languid content. Whatever it is she is smiling about, she is very pleased about it. Of all the time Zack has spent working with Dr. Brennan, and of all the jewelry he's seen her wear, he has never seen her show any piece as much affection as she is this pendant.
He walks closer to her office, making sure not to walk so close that he disturbs her. Studying the silver piece she is still affectionately touching, he realizes that he's never seen this piece before today. And it looks like nothing she has ever worn to the lab before. It's smaller, more delicate, and when he catches sight of what it really is, a religious piece, he becomes puzzled.
Zack Addy knows that Temperance Brennan, renowned forensic anthropologist, does not believe in religion, only science. So, he concludes to himself, this piece must be very special, and from a very special person, for her to wear the religious marker around her neck. He wishes he could observe her longer, but he knows if he stays too long that she'll realize she's being watched.
Just as he's about to resume movement he realizes he's stayed too long. Dr. Brennan has just stirred from her daze and is looking at him. She doesn't appear to be upset that he's been watching her, no, that's not the look she's giving him. The look is one of guilt, like he had caught her doing something and was going to admonish her for it. In a split second he realizes that the thing she feels guilty about is her attention to the necklace.
Zack feels very proud of himself as he offers a half-hearted wave and steps into his office. He has just read Dr. Temperance Brennan, deciphered a moment of pure emotion, something neither of them are good at.