What is the test of probable cause for possession of a prohibited substance?

'The test of probable cause 'should not be understood as placing the ordinary man of ordinary care and prudence and the officer experienced in the detection of narcotics offenders in the same class. Circumstances and conduct which would not excite the suspicion of the man on the street might be highly significant to an officer who had extensive training and experience in the devious and cunning devices used by narcotic offenders to conceal their crimes.' (People v. Williams, 196 Cal.App.2d 726, 728, 16 Cal.Rptr. 836, 837) . . .