unannounced inspection

基本解释

In case of suspicions or irregularities an unannounced inspection may take place at any time.
在怀疑或情况不规律，任何时候都可能会抽检。
ceres-osc.com
In retirement, he had a habit of popping up, particularly in booming Guangdong province, on unannounced “inspection” tours.
退休期间，他还不时露面，尤其在繁荣的广东省，来个突然的“视察工作”之行。
If a restaurant doesn't earn an A during its first inspection, it's considered scored but ungraded and a Health Department official will show up unannounced to re-inspect and possibly award a grade.
如果一家餐馆不能在第一次检查时拿到A级，卫生署只会打分却不会分级，以后卫生署的检查员会在没有事前通知的情况下再度检查，然后以此为基础对餐馆进行分级。

The Bell 430 helicopter disappeared while flying through a heavy rain storm, en route to an unannounced inspection of a local district.