Here is why the police did not arrest the silencer owner (if the OP is correct). http://apps.leg.wa.gov/rcw/default.aspx?cite=10.31.100
Arrest without warrant.
A police officer may arrest a person without a warrant for committing a misdemeanor or gross misdemeanor only when the offense is committed in the presence of the officer…
There are exceptions, but none of them are applicable here.