Seim asked the officer if it was a registered unmarked vehicle for undercover work.
When the officer replied that it was a patrol car, Seim informed him of the law, saying, "You're not allowed to have patrol cars that are unmarked, are you aware of that?"
In the state of Washington, police are allowed to drive unmarked vehicles for special undercover work but not for regular patrols. This is not a national law.
Seim then went through all the normal steps of a traffic stop: taking the officer's name and asking to see his license. Then after a lengthy discussion about the law, Seim let him off with a warning. He urged the officer to speak with his bosses about their illegal patrol cars.
As odd as this may seem, Seim explains that he has a reason for this type of activism. He says that police using unmarked vehicles for regular patrols puts citizens in danger, because people can never be sure if they're being pulled over by actual cops. On his blog he wrote, "If you think it's not a serious issue, try asking those that have been raped or lost loves [sic] ones because of unmarked cars."