Should be on the bottom bracket shell, but ehre aer photo evidences of bottom of down tube exampleas as well, however I tend to think, they might be local additions e.g. coding by police for theft insurance. Don't get surprised if you don't find a serial at all e.g. mine also does not have one, but since I tend to believe, that it was a made to order bike, maybe it makes sense, but even simpler models showup in "help to ID" threads on the forums without one.