[He freezes a bit, hand still clutching at the collar, fingers still brushing against hers (for... what? Some semblance of comfort? She's not his Aoko, but she's still Aoko, and she did help save his life). He had forgotten he had mentioned prison to Conan in his delusional, anemic state... and now it didn't look like he had a choice but to elaborate]

Yeah. It's how they keep track of the prisoners.