Basically when Google employees ask permission to work on a side project, Google asks them to release it open source under Google's GitHub but mark it as not an official Google product. This is probably something the authors work on nights/weekends and not anything to do with their work.
I had no problems getting approval for a tiny 2D home-trainer game I worked on this winter..
I suppose it varies depending on what you do. But if you work on part of it during 20% time, you probably can't do this.
disclaimer: I work at Google.
This is a way to have your cake and eat it too. Most code you work on at work isn’t open source at all.
Alternatively, you could have a side project that’s completely separate from your job.
Google or any other company would have very hard time forbidding what employees do on their free time outside work, regardless of what they feel like putting on the work contract.