I don't think it should matter how the "consultant" gets paid to determine if they should be present at the audit. Just because someone gets paid as a contractor or as a supplier, it should not matter. If a VP tags along for the whole audit and tries to answer questions for every machine operator, it is the same problem.
I am very well behaved consultant during audits. I usually get the standard warning from every auditor at the beginning of each audit, at least until they get to know me. I typically don't answer anything unless I am directly asked. For some of my clients I am part of their system, such as their internal auditor, so I am the auditee on occasion. I had a QS audit yesterday and the only two times I spoke up were:
1. Auditor was claiming that segregation of lots between samples until checked was a requirement. I agreed it is good practice, but not required as you could sort 10 lots instead of 1 if they got mixed up, as long as all are segregated and identified. Client decided to add it to his work instruction anyway, so no big issue.
2. Direct questions about the internal audit I did for them.
I will speak up if I see that my client is "lost" by the question and more or less restate what the auditor asked in terms they will understand. Sometimes I get direct questions like "why did you do it that way". If my client is struggling because they don't know the answer, I let them struggle. I agree they have to be familiar with the system and won't have me around all the time to answer questions. I will also stop my client from arguing if there is a true nonconformance and tell the auditor to go ahead and write it.
I actually asked an auditor to write a minor nonconformance on one of my clients on a QS/TE audit. After six months of myself and the quality manager fighting with the owner to clean up his shop he did not make much progress. He had so much junk stacked up that it was a safety hazard. The auditor was also concerned it was so bad. I felt the only person the owner would listen to at that point was the auditor, so I took him aside and told him I thought it was a good idea to write it to wake him up. Ended up with a closed door meeting with the owner and auditor alone to discuss it instead of a nonconformance.
Overall I tend to find audit days very boring. If everything is going well I usually try to find an empty office to do some other work and check in every so often. After observing so many there is not much excitement, unless I get a "bad" auditor and do have to challenge interpretations.
Tom