One specific comment from an auditor's perspective: While it is true that it is a sampling process, and the auditee is provided with an audit schedule, there are many times when other areas are "touched upon" during the performance of an audit. An example - While reviewing customer complaints (covered every audit), I notice that there seems to be a pattern of late delivery complaints.

As a conscientious auditor, I will probably ask if this was identified as a trend and what is being done about it. If it appears to be ignored, I'll probably make it a point of reviewing that process that appears to be substandard, even if it wasn't on the audit plan.

This often results in confusion and whining

since "that wasn't on the audit plan."
Remember - the audit plan is a guideline. A good auditor will follow the audit trail to wherever it may lead to ensure that the system should still be registered. It should never be a game to try and outthink the auditor to enable slacking off on the system. After all - You wrote the requirements for your system, don't you want them followed all the time?