scret到底可不可数?新概念第三册24课有如下两句话：We often read in novels how a se

scret到底可不可数?
新概念第三册24课有如下两句话：
We often read in novels how a seemingly respectable person or family has some terrible secret which has been concealed from strangers for years.（这里的secret没有加s变复数）

To varying degrees,we all have secrets which we do not want even our closet friends to learn.(这里secret加s变复数了）
感觉两句话的secret都是“秘密”的意思,朗文中查到的secret也是可数名词,为什么第一句话没有变复数?