Nobody goes around just saying they're thieves. That'd be stupid. That's what thugs do. Thugs are about intimidation. They want you to know who they are. Makes it easier to fear them. Thieves have more subtlety. You're not supposed to know a thief's even in the room until it's too late, and sometimes not even then.
That's what Matthew does. While he and his sister Alex generally get by selling minor crops they grow around their home in a village in the Landis Kingdom, he sometimes comes up with more... creative means to earn some extra money, often to his sister's chagrin. He and his friend Jacob have been known to go out on jobs in distant port towns of Landis or other high-traffic, high-value locales. He's yet to be caught for any of this, though if anyone's paying attention at the castle, he MIGHT sort of be identified for abandoning the Royal Army after he couldn't get away from one of King Landis's drafts.
Of course, part of why he doesn't get caught is a firm grasp of his limits. Secrecy and an inconspicuous air about you are key to being a thief; what're you going to do with the largest shipment of jewels that've come into the kingdom in generations? Steal them all and have everyone in the kingdom on edge looking for them? Sell the whole lot and risk having someone recognize them? Keep them at home and brag about them to your neighbors? Of course not. You're going to be silent and not give anyone a reason to think something's up with you. You'll move silently, you'll stay in the shadows, you'll take the small stuff you can get away with smoothly, you won't leave any tracks, and you'll walk back into town like nothing happened. A lifetime of practice has made Matt good at this. He's quite skilled at what he does, he's rather humble and quiet about it, and really, sometimes it's just fun regardless of the haul.
Of course, living that life DOES have its downsides from a social perspective. A significant subconscious level of distrust, for one. Innate cynicism, for another. After all, Phinn's been trying to get through to him for numerous years now. Though, to be fair, things have been a bit hectic lately, assuming he's even back home, so he rarely gets a chance to notice her.