There are not a lot of companies who publish any of the source code online. And why would they: documentation takes time, you need to package it some way and the only thing that can happen is that somebody secretly becomes rich from your work. Worse yet, the company may loose face when security bugs are discovered.
In the meantime, we developers know all know to look at open source involvement when we look at hiring. We know that if you publish code online you are willing to be open for suggestions and criticism from the outside world. Further more, if you are able to get patches accepted, you know how to work with a team you have not worked with yet and how difficult it can be to do it right. Working openly will set you open to opinion, and if you think your code is good, why should you not?