Google: $100M Golden Handcuffs On Wildfire To Retain Employees After Acquisition
techcrunch.com
techcrunch.com
YMMV depending upon where you live but in California what you do on your own time with your own equipment belongs to you, not your employer.
The way I read it is that Google has their hands in so many projects that they would have a good claim on your free-time work, even in California. Just another good reason to ignore their recruiter emails.