Unannounced (adjective)

1. Not previously declared or made known.
2. Not having been formally announced or notified.

Origin:

Mid 17th century: from un- (expressing negation) + announced, past participle of announce.

Examples:

1. The company made an unannounced visit to inspect the facilities.
2. She arrived at the party unannounced and was greeted with surprise.
3. The government made an unannounced change to the policy.
4. The unannounced inspection caught many employees off guard.
5. The unannounced arrival caused a stir among the staff.