同学你好，该知识点来自沪江网校《新概念英语1、2、3、4册连读【7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~
as they soon discovered 非限定性定语从句
disguises can sometimes be too perfect 主句

as 引导非限制性定语从句，代替整个主句，对其进行说明。常用于as we all know, as is known to all, as is it, as is said above, as is often the case, as is reported in the newspaper等句式中，通常均由逗号将其与主句隔开。as 有“正如.....”,  “就像....”之意。
  As we all know, the earth is round.
  As is known to the United States, Mark Twain is a great American writer.

这里as they soon discovered 正如他们很快发现的，发现的内容就是disguises can sometimes be too perfect

as在这里相当于discovered的宾语。

可以在这里看看as引导的非限定性定语从句
http://www.hjenglish.com/new/p1209734/

祝同学学习进步~