Branch audit concern

Our bank currently co-sources the one person audit department with an outside CPA firm. The CPA firm handles all branch audits. We are running into problems with branches not being able to verify who the auditors are to let them in the buildings to conduct the audit. The auditors arrive before the employees (to observe opening procedures) and it creates a potential security concern, because branches are instructed to call the police when suspicious cars are seen in the parking lot. What procedures can we institute that would allow branch personnel to feel comfortable with who they are letting in the branch? I was hoping someone had a similar problem and has already found a solution.

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