Electric Shock off a Light Switch

[a-z electrics]

If cause is static electricity then you wouldn't necessarily need an earthed surface to be touched to allow discharge. In fact there would be a bigger discharge if there was a small insulative air gap . Its possible if the cpc was a (few) mm away from a fascia screw tip then a large build up of static eventually discharged across. I don't think you would necessarily see a visible burn Mark.
Some people are particularly susceptible to static build up just because of their physical chemistry. And I'm not saying they don't wash!