"Picks up on almost kind of active magic. Curses, charms, spells, even magic constructs or sources of power -- though it won't pick up on most naturally magical creatures." That includes mages and other witchers, for the most part. It's weird, seeing the thing around Strange's neck, and Lambert feels practically naked without it, but ... well, he's already pretty much determined Alola is as magicless as it gets. He tamps down the discomfort and continues, reciting from memory.

"Once you get near enough, you can't miss it. It'll feel like it's tugging you in the right direction. Your own magic might throw it off, though." It's spiky and a lot heavier than it looks, so it's also probably uncomfortable. Also, he's not telling Strange that at one point he used it to torture information out of a vampire!