I'm wondering if the phrase itself is also designed to be understood by the fewest number of people possible, in addition to being hidden?
I've heard it very infrequently and usually with some US context in the conversation.
Later the meaning of audit branched/changed, and one common modern sense of the word is to do a thorough examination of something, comparing results against specification. I speculate that it first became a technical specialized legal term (in a context where it was originally still about a hearing in court), and then mutated as it filtered back out to people who didn’t know the word’s origin.
The "audit" in "audit an academic course" (meaning to attend the course without obtaining a credential) branched from the original sense of the word.