Firstly, obligatory invention disclosures are often enforceable if they're related to your employer's business, as they often exist to protect your mutual interests.
Which clearly states that if you're working on the same area your employer is paying you for, it's not going to be trivial to claim it as your own even if you did it on your own time.
In short, California has the benefit of making IP assignment unenforceable if it's on your own time, in an unrelated area to your employer. But it's not carte blanche.
Not a lawyer, but talked to one about this with respect to Google's employment contract. His assertion was that you would be protected if your home project was not in conflict with your duties at the company, even if the company has other branches or departments working on something similar. He referenced some case law which involved an employee at either DEC SRC or DEC WRL who was working on his own thing, started a company and DEC tried to claim ownership based on the 'we do that' variant. Except he wasn't part of the group that did the conflicting work or involved with it. It has been like 5 years now since I did that research but if I can find it I'll see if I can add it to this thread.
Note - the other interesting bit here is that if you're worried you might be at risk, just quit. After all nobody who is suing their employer for ownership of their work stays an employee (at will work clauses). Even though I had confirmed that work on my project could not legally be claimed by Google by the time I had done the research and ascertained that it would end up in a lawsuit I was already convinced it wasn't going to be a good long term fit for me. That simplified the question immensely :-)
Note I did state "conflicts of interest aside". Something search related while at Google or accounting related while at Intuit would clearly fall in that category. A new pet food would not.
Sure, I just thought the suggestion that you don't even need to disclose it, while true in theory, can be risky.
My original post wasnt intended as criticism, it really just trying to provide commentary that while Canada doesn't have the full protections of Cali, the typical contract only assigns specific areas relate to your work.
Also, related, non-competes in Ontario at least are largely unenforceable.
