An announced audit is booked for an agreed date, so the site knows when the auditor arrives. An unannounced audit happens inside a window without a specific date being given. The requirements assessed are the same. What changes is how much a site can stage, and therefore how much the result tells you about normal operation.
Why buyers prefer the unannounced result
An announced audit measures the site on its best day. An unannounced audit measures it on an ordinary one. Housekeeping, glass and brittle plastic control, personal hygiene practice and record completion in real time are the areas where the two diverge most visibly.
What it means for a supplier
Sites that pass unannounced audits comfortably are the ones where the system runs without a pre-audit sprint. If preparing for an audit takes a month of overtime, the finding is not the audit result, it is that the system is not operating day to day.
Practical point
Several schemes now include an unannounced element in their certification cycle, either as an option that earns a marker on the certificate or as a mandatory part of the programme. Check which applies to the scheme and issue on the certificate in front of you.