Nobody put it away. The cupboard where it was found is where it was usually kept.
Yes, I know. So Sheila would have had to go the cupboard, get the silencer out, put it on the gun, shoot everyone, and then put it back in the cupboard before she shot herself. That is generally considered a highly unlikely scenario, partly because people think she wouldn't try to shoot herself upstairs, realise that she couldn't reach with the silencer on, go downstairs to put it away, and then go back upstairs to shoot herself.
However, it's possible that she was not upstairs when she decided to shoot herself.