Just because a carproof, or carfax doesn’t show any accidents it doesn’t always mean the car you’re looking at has never been in an accident or otherwise repainted.
When buying a used car, do as the pros do and get a paint coating thickness gauge.
The reason this tool is very effective in determining paint repair is because OEM paint is applied by a robot to bare metal, and is both consistent and relatively thin. Expect an OEM paintjob to be between 3.5 and 7 mil (90-180 microns), and it shouldn’t range more than 2 mil on any given panel.
When humans repaint a car, or blend paint, or use filler, or do any other kind of paintwork, two things happen: the paint gets thicker and inconsistent. Humans aren’t robots, and they certainly aren’t working with completely bare metal.
These videos give a good breakdown of how this process works:
If you start seeing 10 mils+ or variations in thickness more than 2-3 mil, consider that area to most likely have been reworked in some sort of way.
Some cars are now made with aluminum panels, and getting a paint coating thickness gauge that can do Fe (Iron based) and nFe (non iron based) readings is advisable.