I implementing an auditor role. I have done this by copying the non-editing teacher role and then giving it the course:view permission.
Then for each course this person will audit, I go into Course Administration --> Users --> Other Others --> and then add this person in the auditor role.
This works great in that the auditor now has access to the course and the other participants are not aware of it.
The problem I now have is how to allow the auditor to find the courses that they are auditing. Because they are not enrolled in the course these courses don't show up under "My Courses". These courses are also in different categories. Ideally when this person logged into moodle they would immediately see a list of these courses. Any ideas would be greatly appreciated. Oh yeah, this is moodle 2.2.2. if that makes a difference.