All this time and I never realized that when you hijack a vehicle it literally tells you how many parts said vehicle has on it that you haven’t unlocked yet. I never looked there before.
For every part (paint job, gun, etc) there’s a different indicator actually. Like for wheels you get a small wheel icon, for driver weapon parts, you get a small gun icon displayed. The one you just have displayed, means you have one new undiscovered part, which is a paint job.