But it's not difficult to notice you have a decent-sized spot of your skin missing.
No. He would have slipped into the royal bedchambers, running like water across the room's floor, hidden from the moonlight by a penumbra of subtle darkness. Leaning over the sleeping form of the Empress, he would have traced the contours of her neck gently, lifting the tattoo from her skin in one fluid motion without Kintyra so much as stirring in her sleep. Perhaps he would have caressed her cheek with the back of his hand, as if in parting, before sweeping silently from the room, causing the curtains to flutter as if touched by an errant breeze in the still air of the city.
It's implied that he stole it off her neck without her noticing, and without any physical damage to her lovely neck. After all, Rajhin would never have been venerated as the greatest thief ever to have lived if he had just broken into the royal bedchambers and carved Kintyra's tattoo off with a rusty knife.