It appears Atkins does not include some carb bearing ingredients as they do not get absourbed by the body in the same way as true carbohydrates. (such as Glycerine)

This is an blurb from the Advntage Bar:

** Glycerine, while included in the "calories" count, has been omitted from the "Total Carb" count, as it has only a negligible impact on blood sugar/insulin level.

Here is a site on sweetners which you may find interesting specifically about Sugar Alcohol. These are what I beleive may be some of the hidden carbs.
Sweetners

Sugar Alcohol
This family of compounds, also called polyols, which contain neither sugar or alcohol, is a boon to dieters. The reason: they affect blood glucose and insulin levels much less than real sugar or not at all.