we have the same problem you do. when you sold the stock you may have reported the proceeds (including what you got from CA) for tax purposes in that year. in this case, since you already reported them the proceeds would be non-taxable. they would have been held in the brokerage account until inactivity required the proceeds to be turned over to the state. that would make them non-taxable. on the other hand, the proceeds may be the result of a lawsuit or maybe a contingent payment. under Ca law inactivity in an account for three years would make them subject to the unclaimed property laws. so those proceeds were received by the brokerage at least 3 years ago.
if you know the brokerage firm contact them. it's up to you to determine taxability.
how much effort you put into finding out more is sort of dependent on the amount involved.
