I tend to think that an audit that raises nothing provides no value at all. It is similar to the test team. I expect that many things will be covered off by the teams leading up to the audit (or testing in the comparison) but I only believe they are being thorough when they raise the first non-superficial issue. No issues (defects) means that it hasn't been evaluated (tested) enough.
...and, basically, David is spot on in his assessment.
-- Stop your grinnin' and drop your linen...they're everywhere!!!