The answer, and what it costs to be wrong
We used an audit firm once and it went nowhere
That is the common experience, and the reasons are structural rather than a comment on the firm.
What usually went wrong
Traditional engagements start at the formal audit, which is the most expensive and most restricted step in the lease, and they need a portfolio big enough to justify hourly work. They arrive after the window, they cover one year, and they leave nothing behind that checks next season. We start at the desk review, which the lease does not restrict, and the engine reads every site at once.
One year, once
The error that costs the most is the one that repeats: a clause misread on a lease template, on every site signed on it, in every year since. A single engagement on a single year cannot see it, which is why the same money is still there.
See what a portfolio pass returns
Scan five representative sites. You will see whether the same finding appears on more than one, which is the question a single-lease engagement never answers.