Don't have one on hand but I have seen quite a few. I have heard that they are mostly 20% time projects though I'm not sure if they try to take code written on your own time but it looks like they do (and anything else you have written before working there) from this:

https://opensource.google.com/docs/iarc/

> As part of your employment agreement, Google most likely owns intellectual property (IP) you create while at the company. Because Google’s business interests are so wide and varied, this likely applies to any personal project you have. That includes new development on personal projects you created prior to employment at Google.

Just to be clear, it says "new" development while you're employed by Google, they don't claim to retroactively own your prior works pre-employment. I'm sure they ask you to declare it though so it's clear what you built before joining.

Perkeep is written/founded by Brad Fitzpatrick (google/golang engineer) and you have to sign a google CLA to contribute to it: https://perkeep.org/code#contributing