How have the courts interpreted the Miranda warning?

During the investigation, defendant was detained on a "pretext stop" because the truck he was using had paper license plates. When the officer asked him why he had no metal plates, defendant said he did not have them because the car had been repossessed a year prior. After defendant gave the officer permission to search his truck, the officer then found the official license plates under the seat. On showing them to defendant and asking why they were not on the truck defendant said "he was going to sell th[e] truck." Defendant argues counsel should have moved to exclude these statements because no Miranda warning was given. However there is no evidence defendant was in custody and no warning was necessary. (People v. Farnam (2002) 28 Cal.4th 107, 180 [inquiries during temporary detention not subject to Miranda].)