If the user is performing a Custom (advanced) Search, and they do not check any checkboxes for a particular section, the resulting behavior is to locate records as if the user had selected ALL checkboxes for that category. But it's not at all clear to the end user that that is what is going to happen.
What would be nice would be an "ALL" or "I Don't Care" checkbox at the top of each checkbox category. When checked it would, in turn, check all the underlying options. This gives the user a warm fuzzy feeling that the system is going to do the right thing.