I wonder if one could train a cat to lick envelopes. The FBI DNA testing people would have some interesting meetings with the agents.
That sounds like one of the harder ways to get cat DNA on an envelope.
Of course you are voidnull. We could tell by the scratches on your forearms.
If they are already suspicious of you, then you've already lost. In the above scenario, voidnull's dna or fingerprints would have to be cataloged for them to find him.