A similar mark is used in an unrelated game Black Ops 3 zombies Shadows of Evil, all 4 characters have a mark not unlike the ones we see in The Witcher 3.
Is it a coincidence that Gaunter O'Dimm marks his victims or debtors or debt collectors with a very similar kind of mark the Crones of Crookback Bog use to indicate to other evil beings that if a person or thing has that mark it belongs to them, and is a test of trust as when it is tested it can inflict horrible pain.
