Every SV company I've ever worked for has explicitly disclaimed rights to my work done off hours with my own equipment... as they are required to do under CA law.
"A typical silicone valley company's contract usually have some clause saying that all your creative work while contracted with the company, even on off hours, belongs to the company."
My experience, working for a number of silicon valley companies, has been the exact opposite. There is a clause that says (in legalese) "we don't own stuff you do on your own time with your own equipment if it isn't related to what you do at work"
It is my understanding that California public policy actually requires companies to put this disclaimer in work contracts.