The unannounced audit programme is for operations enrolled in certification Options A1, A2, C and D. Each year the Certification Body will choose 10% of its clients for an unannounced audit; therefore, over time, everyone will have an unannounced audit.
If an operation is enrolled in Option A1 or A2 (4-year audit cycle), there is no change to the way the audit cycle or random audits work. The operation will still be informed in advance if they are randomly selected for an audit. However, they may not be told the exact date of the random audit (i.e., it could be unannounced).
More detailed information about unannounced audits can be found here.