Maybe it wasn't there when the police searched the scene
Maybe the murderer came back and replaced it after the search had been completed?
Okay, let's run with that.
You're the killer. You climb out of WHF's kitchen window, heart going like the clappers, run to the estuary and grab you mother's sit up beg bicycle that you left by the - crumbling, twisting, pot holed, well rained on and muddy - sea wall, strewn with clumps of grass the size of sleeping policemen.
Then you cycle like a bat out of hell...right over the edge of the sea wall onto the rocks below. When you come round the bike's had it, so you run the rest of the way home.
No..?
Okay, you walk home. Forget the fact that the police's own timing of how long this would take showed it would leave you impossibly short of time to call the police when you need to.
Unsurprisingly, it's not until the next day that you notice you left the silencer in the pocket of the trousers that you changed out of the previous night. Gawd, the police'll know she was murdered because the silencer's missing. So you wait until nightfall and return the silencer to WHF. You hide it well back in the gun cupboard.
But later, at home, you ask yourself, "Why did I do that? The police have already searched the place, they'll know Sheila was murdered, I have to get it back!"
So, off you again.
Or do you? Because part way there you're conflicted again. Should you take it, or should you leave it?