At a guess, he didn't want it to be known that it had been used?
Ah now, ah now, ah now, Jane!!! This is where we start going round in circles again!
I thought the claim was that he was going to leave the silencer attached to the rifle, but when he laid it onto Sheila and realized it was too long for her to commit suicide 'only then' was he forced to remove it and hide it in the gun cupboard??