True. I'm less and less confident in the credibility of drug tests. Random as possible is helpful but with lack of resources and other restraints this becomes difficult. Another is cost ( at least for the majority of agencies). Federal POs have a patch that is worn for a week or two at a time which is supposedly pretty good and somehow guards against tampering. Hair folicle and ION scanning is also expensive and less frequent/available.
I guess something is better than nothing but still....