Do you understand that the water stays on the property and can be collected? Really, this practice survived jurisprudence.
Do you know how plumbing works?
Do you think there is a valve on the pipe to the street?
Yes it is true, they find the residue in the pipes and bowls etc. You could probably flush a couple of kilos but not many kilos. If you are moving many kilos, you probably don’t live in some run down apartment. Maybe you store your stuff in one and let your underlings take most of the risk.