The point of unannounced audits is to see if you are doing what you say you do all the time or only on the days you expect the auditors to be there and more importantly to understand the true operational practices of the site.
And - If the company needs unannounced internal audits to "...understand the true operational practices of the site..." the same is true - The company has bigger problems than anything unannounced internal audits will reveal.
Unannounced audits are never acceptable in my opinion.