We don't have anything written down but this is what we do.
The receptionist checks the person's ID and gets them completing the health questionnaire then calls one of the Technical Management. That person then sends out a mail (we have an email group for audits) to let everyone know the auditor is on site, they then tell the QA team who scatter across site to all production areas to check for "sillies". That manager then picks up the HACCP plan and then goes to pick up the auditor and takes them to a free meeting room. Once in that room they find out the auditor's shoe size then calls from that room (there's no point hiding it) to the first operations person on the tour ostensibly just to tell them the shoe size but it's a second chance to make sure that person knows the auditor is on site in case they've not seen the mail. We also get any nutritional requirements in that opening meeting so we can make sure the lunch we're arranging in the background is to their liking.
One thing we need to get better at but it normally happens is we have a "background" person who makes sure the trace is done etc. They will normally start bringing files to the audit meeting room which are likely to be needed later, e.g. pest control, calibration, internal audits, TACCP etc. That means if the auditor comes back later and there's a lull in proceedings, they can start going through some other stuff. We also make sure we bring EVERYTHING when there's a trace, e.g. associated GMP audits, glass audits, specification, ingredients approval etc etc. Normally it then means we can cover off most of the audit requirements without then scrabbling around for bits of paper.
My key piece of advice with all audits, not just unannounced is keep the pace going, keep moving, keep having stuff ready. It makes you look professional, it makes the audit go more smoothly, there are no awkward silences and it normally makes the audit go more quickly. The best way to do this (apart from the tips above) is to ask the auditor regularly of a list of what they want to look at next. This means that you can get that ready for them while they're going through other things.