It was at that point I was was convinced that emailing legal to ask permission was a hack to get you to disclose early what you were working on so that when they sued you they already had some of the initial leg work done :-(.
I consulted with an attorney on this and they advised me that because I was working in California the burden of proof that it was related to Google's business was on Google. But that in his experience they were not above pushing the edge of the definition. The bigger point though was that people don't work at companies that are suing them and generally don't work at any company if they have a reputation (real or imagined) of doing work on their own time that is "against" their employers interest.
The summary of his advice was never, under any circumstances, work on any project you don't want to give Google ownership of while you are employed there. And when you have an idea that you want to develop, quit so that those parts of your employment agreement are nullified.
They can still come after you if they think you are using "proprietary or confidential" information in your new effort but that is less common and it makes them look like the bad guys not you.