It sounds like our disagreement is only about whether it is "really weird" for an officer to request a passenger's ID during a traffic stop. The City of Denver has this to say on the subject: "Depending on the circumstances, officers may request identification from passengers as well." http://www.denvergov.org/police/PoliceDepartment/TrafficEnfo...

And as you say, the officer probably found it odd that there was a passenger riding in the back of an apparently private vehicle.