Washington State has no laws concerning silencers other than;
RCW 9.41.250 Dangerous weapons.
(1) Every person who:
(c) Uses any contrivance or device for suppressing the noise of any firearm,
In WA a silencer is not even defined as a firearm. It is not illegal to use one on an air rifle, although I think they are only effective on those air rifles using a high pressure pre-charged reservoir. So in other words, unless a WA State police officer has reason to believe that a shooter was using the silencer on a firearm, then there is no reason to confiscate it. This is especially true if they are shown the registration paperwork, be it an ATF form 1, 2, 4, or 5.
If what the poster in the link above said it true, then the police officer that took the silencer had no reason to do so and may be in violation of federal law.