This look makes me think of the human phrase "guilty pleasure". She sure derives a lot of fun from getting a bone and exercising her jaw. I doubt that Ziva feels any guilt over eating or other enjoyment (Why should she? Why should we!), but there's something about the look at this moment, as if she's acknowledging that I caught her in the act.
The stripes on Ziva's back seem to have become more prominent as winter progresses. This may have a cause related to the weather, or may simply be part of her maturing. We'll have a better idea when spring rolls around.