I'll assume you've already done an announced audit then. If your very first BRC audit was completed say 1 Feb 2022 and you decide to go to unannounced on your next one then it will always be tied to your first audit i.e., an unannounced audit can happen anytime before the 1st of Feb 2023 but most likely within 4 months.
Once the auditors turn up to the site, you have 30 minutes to get them into the factory to view the most critical point in yout process i.e., your CCPs. Auditors will usually ask to see a check being completed and collect any check sheets or write it down, before you go back to the meeting room and do your usual audit protocol of welcoming them to site, maybe doing a presentation etc. I lived far from my site at the time, so 'key members' don't need to be present (it would be highly preferred) but a nominated deputy can be present to show them into the site and get them into the factory. Key members do need to be there for the opening and closing meeting though.
The whole point of an unannounced audit is that it is unannounced therefore any form of a heads up doesn't really make sense. The closest thing I've ever gotten to an 'announced' unannounced audit is an email from our certification board saying that they're sending a newly qualified auditor with a lead auditor supervising them as part of their quality checks, and then they turned up the same week. Other than that I would just keep a lookout when the 4 month window is approaching and they still haven't turned up.
It is highly recommended that you do not refuse a BRC auditor entry, as it can result in your certification being suspended until a new unannounced audit is completed.
Source: completed the conversion course for V9 and work with unannounced audits.