How Did She Do That?

I found Penny's identity disc in her bedding along with ...



... the remains of the reasonably strong split ring which had connected the disc to a D ring on her collar.  It must have taken quite a lot of force to open it out that much. My guess is she caught a claw in it while having a scratch. In that case I would have expected her to kick up a fuss as she has a very low pain threshold.