The parking placards come from the Department of Licensing, so you can probably call them at (360) 902-3770 or drop it off at one of the offices: https://fortress.wa.gov/dol/dolprod/vehoffices/DisplayOffices.aspx?County=17 Each pass has a # on it, so they'll know who it belongs to.
If someone does contact you to say it might be theirs, when the placards are issued you also get an ID card proving that it's yours. So make sure you get a match on that card. Mine has my name, date of birth, expiration date of the placards, and the ID #s on both of my placards, so along with my license to prove that the name/DOB are me, I'd be able to prove whether it's mine or not. (It's not mine, by the way - both of mine are accounted for - just letting you know how to ID it in case somebody contacts you.)