The NC is legitimate. Technically the audit was not over - closing meeting is part of the audit.
This is bad advice. If a nonconformance is found during an audit, it is written up. Presenting a nonconformance after the fact in the exit meeting is a no-no.
The above is what the auditor reported and I agree it's not a major rather a minor but it's been issued as major.
Here is the issue as I see it. When a nonconformance, major or minor is identified it is written up. The closing meeting is not the time for an auditor to bring it up. It should have been brought up during the audit. It is totally inappropriate to "spring a nonconformance" on a company being audited during the closing meeting, whether the nonconformance was major or minor.
If an auditor DID do that, I would stop the meeting and go to where the nonconformance was identified and do the writeup.
The closing meeting is to discuss findings identified. It may be "part of the audit" in the strictest sense, but to allow a nonconformance to be brought to light at that time brings the question: Why was it not identified when the person/area was actually being audited?
IF that type of "Oh, and we have this..." comes up in the closing meeting then there would be no need to identify ANY nonconformance until the closing meeting.