The defence narrative at trial does support Sheila putting on a silencer while everyone slept. Prior to shooting everyone in bed.
Nevill getting downstairs to phone Jeremy.
This would mean it is a mixture of June & Nevill's blood in the silencer.
However after realising Sheila could not shoot herself with the silencer on, she would have just left it next to her. Espescially if the raid team were inside WHF!