How can the police possibly know whats part of a crime before its been examined by the forensics? Thats why they collect items to send back. They cannot know such things in advance in any case. Such argument is thoughtless.
"The first officer to arrive at the scene of a crime is responsible for taking steps
to preserve and protect the area to the greatest extent possible. The officer
should not let his or her guard down; the scene should always be treated as
though the crime were still occurring until it is proved otherwise.
First priority should be given to obtaining medical assistance for individuals
in need of it. Steps must be taken by the first responder to exclude all
unauthorized personnel from the scene and keep an accurate log of who enters
and exits the scene and the time at which they do so.
Once the scene has been secured, with the help of others, a lead investigator
will start the process of evaluating the area. The lead investigator will immediately
gain an overview of the situation and develop a strategy for the systematic
examination and documentation of the entire crime scene."
It isn't thoughtless. It's actually logical. You refer to them not letting their guard down. Their guard was already down. Jeremy had given them clear and succinct detail of Sheila's medical history. Jeremy had practically told them what they'd find. They knew that no one else was involved. All the family, other than Jeremy, whose belated arrival they'd witnessed, were in the house with a insane woman who had full working knowledge of guns, and a gun in her hands! We KNOW they were wrong to accept his word, but the fact remains, they did.
To get back to the silencer. There is still an ongoing debate about whether or not she could have shot herself twice whilst it was still attached. At a guess, as there wasn't one ON the weapon when it was found, it would hardly have occurred to them that she'd used one, if only because there wasn't one to be seen. IF she'd used such to shoot the others, sooner than make her own demise more difficult/take longer than necessary, she'd most certainly have removed it, prior to taking the shot, and it would most likely have been found close by.