But hidden alongside his legitimate dealings, Madoff was running a scheme that mirrored Ponzi's.

参考译文：隐藏在他的合法交易下马多夫正在策划一个反映庞氏骗局的阴谋。

解答：以上hidden alongside his legitimate dealings为过去分词短语作状语.修饰后面谓语动词，alongside 为介词用法，相当于介词under his legitimate dealings.

类似以上过去分词作状语句子如下

Hidden behind a bush, a hunter was spreading his net to catch the hare.

一个猎人藏在一个灌木丛后面，正在撒开他的网去抓住那只野兔。