In fact, Google has one of the most permissive IP policies that I've seen at any company except for VA Linux Systems. The default is that the vast majority of work done by an employee can be released under an open source license. Sure, there's a process that you have to go through, and unless what you want to do intersects with work that you are doing for your team that isn't intended to be released publically, or competes with a critical company initiative, the default answer is that it will be allowed to be released.
This is not true for many, if not most companies, and as a responsible open source maintainer, I make sure that people understand they have permission from their company before I accept their open source contributions. Otherwise, I would be putting them, and the project, at risk. (Note that because of this, there are many employees which are not allowed to participate in open source development, because their employers will not give them permission to release code written by an employee under an open source license, whether written "on their own time" or not.)
At Google, once you get this (mostly pro forma) permission, you can work on open source projects on your own time, or on 20% time, using company laptops, or using GCE resources, etc. When I was hired at Google, my manager and I cut a deal such that I could work on Linux Kernel and community projects for 50% of my time, instead of the normal 20%. Sure, the overlap between what was good for Google and what was good for ext4/Linux was pretty broad, but Google also paid for me to travel to various conferences in Asia, Australia, Europe, etc.
Also unique to Google is there is a process by which you can get permission for you to out-and-out own the work done on your own time, as opposed to Google owning it and releasing it under an open source license. If you take that option then it really has to be done on your own time, and must not use any company resources, meaning not on a company laptop, or on a company network, etc. This is rare, and I'm not aware of many other companies that give employees that option.