Destruction of the evidence
I was on a bid for the previous Census (2011). I was told that after the forms had been processed everything was destroyed. Whatever was in the envelope, would be completely and utterly destroyed. I did wonder what would happen if they received, say, human remains (legal obligation to report to the Police if they could be less than 35 years old), valuable items, or just important documentation - like someone's will. There is a mediocre detective novel in there somewhere.