The notion that the silencer was contaminated with blood from the underwear is absurd! I toyed with the idea but dismissed it as desperate. If the silencer was faked, it had to be the police.
David prefers to suggest the relatives -
Dipped their hands into a bucket of cold water.
Pulled out some soaking wet underwear which may have had some diluted blood on.
Waited until this underwear dried.
Expertly transferred any miniscule dried blood embedded into underwear into the silencer baffles.
Oh, and then scratched the aga.