So, here is how it works… They test at the main outlet for drugs, then start moving up the pipe and at each junction they mark it “clean” or “dirty” and keep moving upstream on the “dirty pipes” to the individual homes. When the arrive at your back door they have probable cause and you are busted.
The same sort of methodology will find gun owners (gun powder residue) and Jews In The Attic (DNA markers) and can also be applied to your trash at the landfill and the air downwind from your community.
This opens a new form of blackmail:
“Be nice to me or I’ll pee in your toilet.”