Ouch--

*In rummaging, one of the cameos bit Sirius' finger, and he shakes the thing off and sticks his finger in his mouth. With his free hand, he fishes a bright purple ministry-issue confiscation bag out of his pocket, inverts it, and picks up a cameo.*

This one. It opens up, I think, something's inside. Don't fancy finding out what right at the moment, but I think it's what was setting it off.