Sharpie
From Tardis Wiki, the free Doctor Who reference
Infra-red analysis of the Mona Lisa in the Louvre uncovered the fact that "THIS IS FAKE" was written in Sharpie underneath the paint. (TV: City of Death)
Behind the scenes
An illustration of a Sharpie.[1]
A piece of Redacted fanart officially acknowledged by the Doctor Who Twitter account featured an illustration of a Sharpie.[1]
