The State of California is so obsessed with taxes that it has three separate tax departments. The Franchise Tax Board whose domain is income tax enforcement; the Board of Equalization that handles sales taxes; and the EDD, Employment Development Department, that takes care of collecting UI and SDI payments. Each is separate and independent of each other.
As Deb says, the FTB is very tenacious, like a pit bull that won't let go. I've had clients that have had to file returns as long as 23 years ago (there is now a SOL of 20 years). other clients that moved out of CA to a non-tax state and were pursued there because the client had not severed all ties to CA, such as forgetting to have their name taken off the voter registration list, or having a PO Box in CA, or not being able to show proof of having their drivers license, or vehicle reg. in the new state.
To assess some non-filed returns, the FTB makes up imaginary income numbers. Sometimes the mortgage interest paid is mulitiplied by 4 and that becomes the income figure, which is then taxable. Other times, they will take the average income of specific types of businesses, such contractors, insurance sales, etc., and base the assessment on some multiplier of that. At least the IRS use real numbers for such purposes.
One good thing the FTB has done since the first of the year, is to adopt the IRS National Standards for living expenses in calculating collection potential.