Not sure why you think that. Here's California's moonlighting law:
"Any provision in an employment agreement which provides that an employee shall assign, or offer to assign, any of his or her rights in an invention to his or her employer shall not apply to an invention that the employee developed entirely on his or her own time without using the employer’s equipment, supplies, facilities, or trade secret information..."
Engineers always get this wrong. They always quote the first part, and then say "see, anything i do on my time with my own stuff is mine".
You forgot the super-important part, which are literally the words after what you quoted, which say:
" except for those inventions that either:
(1)
Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer; or
(2)
Result from any work performed by the employee for the employer."
#1 covers pretty much everything tech related that tech related people do. For example, if you work for IBM, i doubt you can find anything not related to IBM's business, or the R&D of IBM. Becuase it's not what you think IBM's business is, it's what IBM think it's business is. It also doesn't matter whether it's a secret project you don't know about, or historical, or anything.
It makes no difference what you do for the company either.
If I work as an IP lawyer for Google, and Google builds self-driving cars (among other things), any work i do on my own time related to self driving cars is Google's.
Note also "related to", not "exactly the same as". So if it's in the same area, field, etc, you are screwed.
But you don't have to take my word for it, look at court cases in CA, which find the same :)
Now, maybe if you work for a small, super-focused company, you may be okay.
But good luck if you work for any mid or large sized tech company, which often have so many projects, strategic initiatives, and research that it covers pretty much everything in the world.
To wit:
In every case an employee has come to me saying "well, we don't do that, so why would you own it", it has been a case that the employee did not know there was a team doing it.
I have never had a case of an employee coming to me and it turned out "yeah, you know what, we don't really do anything in that area"
"Any provision in an employment agreement which provides that an employee shall assign, or offer to assign, any of his or her rights in an invention to his or her employer shall not apply to an invention that the employee developed entirely on his or her own time without using the employer’s equipment, supplies, facilities, or trade secret information..."
http://www.weblaws.org/california/codes/ca_lab_section_2870