Some auditors do not audit by results or objectives, they want to see a documneted plan in writing. Some are happy with the schedule, and evidence that you are adhering to it, which is proof you have a plan.
If you wish to appease the auditor you stated, then the plan should stated in your quality manual. It is should state something to the effect of what areas affect quality and the frequency in which they will be audited. The determination of what will be audited by whom, how they are prioritized and the scope of those audits.
Here is an example of a documented audit plan: "All areas stated in this quality manual as affecting quality will be audited no less than once per calendar year by personnel deemed competent per the requirements of section (what ever your competence awareness and training section is).
The determination, scope and prioritization of audits on the audit schedule are based on impact to the company's quality system, findings found in internal, customer, 3rd party audits. the company's strategic mission and the subsequent area's alignment of objectives to the company's mission. Responsibility for the audit plan and schedule are that of the company's quality Manager or appointee"
....and wallah you have a plan that is documented. See ISO9001:2000 8.2.2 "The responsibilities and requirements for planning and conducting audits...shall be defined in a documented procedure."
What is the difference between an audit plan and an audit schedule?
The audit schedule is specification of the audit program. It basically identifies the individual audits you will be conducting for specific areas. It will identify the frequency of the audits, the auditees, the high level audit criteria (e.g. the standard used as reference). At a company where I am currently working we have an annual schedule for both our internal audit program and our supplier audit program.
The audit plan is a specification of the planned activities for a specific audit. It will identify the names of the auditors and the auditees, the audit dates, it may contain a predefined audit questionaire, which was drafted during the document review in the preparation for the audit. The audit team should follow the audit plan while conducting the on-site audit activities.