Here is the section:
(b) No passenger in any motor vehicle who is under the age of 21
years shall knowingly possess or have under that person's control any
alcoholic beverage, unless the passenger is accompanied by a parent,
legal guardian, responsible adult relative, any other adult
designated by the parent, or legal guardian for the purpose of
transportation of an alcoholic beverage, or is employed by a licensee
under the Alcoholic Beverage Control Act (Division 9 (commencing
with Section 23000) of the Business and Professions Code), and
possession or control is during regular hours and in the course of
the passenger's employment. If the passenger was unaccompanied, he or
she shall have a complete defense if he or she was following, in a
timely manner, the reasonable instructions of his or her parent,
legal guardian, responsible adult relative or adult designee relating
to disposition of the alcoholic beverage.
So, when you go to court your defense would be that you did not KNOW it was there. if you can cast reasonable doubt on that knowledge, then the elements of the offense may not be met and an acquittal may result.