I have been here for over 6 years now and I have never heard of 'hidden' names.

No, but they will have nicknames given by there family.

Or authors and famous people often have pen names.

Yes, even my ABC (American born Chinese) friend doesn't have a middle name.  On his birth certificate I believe his Chinese name is his middle name, but he doesn't know his Chinese name, haha.

Nicknames are common.

Dont know if this is what you mean, but often times, family and close friends will "double" the given name as a form of endearment.  So, my friend "Chen Bing" will be "Bing-Bing" to his family and really close friends.

I'm positively sure they don't have middle names. When people saw that I had a middle name they asked me is your name this or that. I said this is my first name that's my middle name and the one at the end is my last name. 

Then I was told that in China they don't carry middle names only first or given names and last names. The middle name is usually a christian name anyways and I believe most chinese are not of that religion. Anyways, I'd say no!

No. In the past, scholars and the upper class often had 2 other names. But no one does anymore.

The way it worked was, when they were born they were given a name 姓名 that's a surname and given name, and that was the name they would use most commonly. The upper classes, scholars, litterati etc. also had a second name (字, zi) which was usually related to their given name, like an explanation of it. When they were older they would often give themselves a nickname/pen name/alias called a 号 hao. This usually had nothing to do with their name and is kind of similar to the way we choose a name for ourselves on this forum.

Yes, but only very occasionally. 

I think I had a schoolmate who hid his middle name in primary school or middle school.

In some Chinese families, their first name is surname and the middle name is "generation name". Like my aunts all share the same middle name. From what I know there are several certain "generation names" in a family in a certain order. After they're used up people will start with the "first generation name" again.

So I guess the schoolmate had a hidden middle name was because he was given that middle name by his family just to fit in with others, but his parents felt like to not use it.

Hope this is interesting for you!

ohChina summed up what I was going to say. My husband has one of those "generation names", but he never uses it and it isn't in his hukou. It's just there, in the back of his mind, but never talked about anywhere else.

GuilinRaf is right. We all call my wife "Juanjuan," or "Pangpang" (fat fat).

There are also style names (字), which are ancient and have gone out of... style. I'm unaware as to whether or not anyone still practices this, but it wouldn't surprise me. Personally, I like it and think it should make a come-back, along with the ancient dresses. Modernity is such a drag!