Found it.
"The mark is the entire length of Drayce's left arm from shoulder to wrist. Unlike most Champions, Drayce finds little point in hiding his mark because his identity as a Champion is already known."
Now, I know what you mean about it being hard to hide, but it makes no sense to have it written this way. For one thing, his identity likely isn't known all over Tamriel, so slipping a sleeve over it would at least help to prevent anyone new from discovering him. Besides which, he'll be traveling with the rest of us. If he's discovered, all of his contacts (meaning us) would come into question - and would likely be killed, whether they're also Champions or not.