Hmm, that's a good question... I'd say if they focused on him and expected to pick up any magic, they'd possibly get a small response. But it's pretty inert except when he shifts, so the magic would be stronger an hour before and after the shifts probably. Then they'd almost certainly pick up something strange, even if they can't pinpoint it. If they know how to identify spells they'd probably be able to tell what it was, or close to, since it's a little skewed. But if they don't, it might seem a little more like he's picked up/brushed against/ate/whatever magic than that it's coming from him.