<p>near the top of the code i see things like, </p>

<p>btn_dropdown._visible = false;
mcMenuBkg._visible = false;</p>

<p>but I can't find these assets anywhere in the library or in any code, how does this make any sense?</p>

<p>The movie clips in the library that look the same have different names and I can delete them entirely and they still show up when I compile and run, or I can add trace statements into their code and they never get called.</p>

<p>where on earth are these assets defined?</p>

You can create movie clips with code dynamically.
This means that you may not have them in your assets if you are unable to find them.
You can create any type of symbol using a constructor out of thin air with actionscript alone.
I would search the code for one of these
```
```
var mybutton:SimpleButton=new SimpleButton();
```
```