Because he's just looked around to give Sabine an unimpressed look--thanks so much, he is not a medallion guy, but thanks--Bellamy gets a full look at her face and the shift of her expression. He knows its look. Not on her, but on other faces, a recognition that leads right into anger.

"Whose is it?" He doesn't bother to ask if she knows it, because she does, and she's already looking around. Bellamy stands, having crouched to examine the chain. Without knowing what he's looking for, he, too, looks around, and his hand goes warily to his sword again, even though immediate attack is unlikely.