After reading feedback and thinking about this a bit more, I decided to delete the post. Sorry.
I would report it to Parking Enforcement @ 503-823-5195 since it’s not legal to have a car without registration on the street. Slower response is using pdxreporter.org to report it. Even if they bought it during the pandemic they would still be required to have the temporary tags in the window (or somewhere easily visible), even if they expire.
Side note: I’ve seen plenty of vehicles with dark tinted rear windows where you can only see the temporary tag in the window if you’re 1 foot away. It’s not legal, but it’s how lazy people interpret the instructions.
Also, you should always be able to read the VIN through the front window on the driver’s side dash.