There are two issues re the silencer which caused the prosecution to decide it had been used. One was the red paint on it from under the mantelpiece, and the other was the presence of human blood which contained an enzyme which Sheila possessed. There have been attempts to discredit both of these issues with no success so far.
If Sheila could have reached with the silencer attached, there was no reason for Jeremy to put it back in the cupboard. Would he even have noticed at the time anyway?