s. without warning or announcement
a. 未经宣布的
1. The teacher arrived unannounced to check on the students.
老师没有提前通知就来了，检查学生的情况。
2. We had an unannounced visitor at our house last night.
昨晚我们家来了一个没有提前通知的访客。
3. The company conducted an unannounced inspection to ensure compliance.
公司进行了没有提前通知的检查，以确保合规。