Most heads have a machined cut below the seat from the factory. This is a grey spot but I work that area on most heads. If you try to put in several angles under the seat, i'm sure it would never fly in tear down.
The certification crew is getting very good at spotting worked ports, especially in common engines. Given the fact they have maximum CC specs for ports, the first tip-off is when the intake surface is machined back to the valve cover bolt holes.
I've heard it more than once, "stock class is where guys spend $5,000 to hide a $250 port job"