Using the MOBI UNPACK utility that I mentioned earlier, you can “unzip” a MOBI file, whatever was used to create it, as long as it’s unencrypted and regardless of whether or not the MOBI file includes its source file.
When I open April Hamilton’s book, From Concept to Community, which I purchased from Amazon in March, 2011, I get a folder named mobi7 inside of which is:
There is no source file.
When I open one of my own books, Anyone Can Make a Kindle Book, which was created using Calibre, I get the mobi7 folder again, inside of which is:
Again, there is no source file.
However, when I open another of my own books, MEMOGRAMS, which was created from an EPUB file using the latest Kindlegen, I get two folders, mobi7 and mobi8, and a file named:
There’s the source file, which I can further unzip if I want, and I get:
kindlegensrc (folder) in which are the files:
But I still have the two “mobi” folders to look at:
In the mobi7 folder I get:
In the mobi8 folder I get:
All this is to say that it is possible to “unzip” a MOBI file. It doesn’t need to include a source file to do so, and it can be done even with MOBI files created with Calibre.