I coordinate the audits for 11 of our plants. Luckily I'm not the one on the hot seat, but I aid in scheduling them. I have gotten a schedule in advance with no date about a month before they showed up, on the morning of the audit after he was in the building, and one auditor sent the schedule with the date included (that one needed to be rescheduled because it HAD to be unannounced).
We have a few sites that we coordinate their audits to save on travel charges. For two of them the auditor showed up on Thur. at one site for a two day audit and not many people were surprised when she showed up on Mon. at the other plant. We've also had them show up with absolutely no idea they would be there. One site had a two week black out date and the auditor showed up the first day they were back open which also happened to be the first day their Quality Manager was back from vacation.
If you are truly audit ready all the time you will be fine. None of our sites have had poor results during an unannounced audit. Some have actually done better on the unannounced audit than on the scheduled ones.
Best of luck you you!