同学你好，该知识点来自沪江网校《英语零基础直达大学六级【8月班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
Police are wary of itinerants who cannot be traced to a recognized camp boundary or to four walls.
to与trace搭配，be traced to追溯到……
These quotations cannot readily be traced to their sources.
这些引语很难查出出自何处。

four walls 四面墙，其实就是房子，房间，在这里指的就是住处。比如
I should hate to be confined within the four walls of an office all day.
我讨厌整天给圈在办公室的小天地里。

警察对那些查不出有固定营地或住处的游荡者保持警惕。

祝同学学习进步～