plain clothes man

例句：

1. 1.The plain clothes man was able to slip in unnoticed and gather information.
   这位便衣警察能够悄无声息地进入并收集信息。
2. 2. The plain clothes man was the first to identify the suspect.
   这位便衣警察是第一个识别出嫌疑人的人。
3. 3.The plain clothes man was able to observe the suspect's behavior without being detected.
   这位便衣警察能够观察嫌疑人的行为而不被察觉。