So, assuming Matt's mark isn't in a place where it's easily seen, if Claire's too professional to bring the mark up and Matt generally avoids thinking about it then...maybe Santino's the one who finally spills the beans?

Like, Claire's working on Matt but needs an extra pair of hands. Santino's there so he helps under Claire's direction. But he's still a teenage boy, so when he sees the mark he blurts it out.