There are many different types of audits which are performed and make up a big chunk of my MS work.
I perform retail price audits in airports in several states. These are open revealed audits with sign-offs from managers to ensure that my checks are completed correctly. They pay (well) by the project and are conducted quarterly. I am badged as an airport employee which enables me to go through employee security checkpoints at the airport.
In addition, I perform program and/or price audits at various medical and hospital campuses that pay by the hour. These audits last anywhere from one to three days and staff are aware before I arrived. The audits are part of the credentialing required at the facility.
Often price audits at pharmacy and grocery store location are considered vendor audits and are not secrative at all since warehouse pars are often required.