"Well, in that case it seemed to be Tanyuu's... abilities that set it off, though I can't claim to know the details." It's not like he's pushed his adoptive sister for the details of the powers that the Void forced on her, for... obvious reasons.

"It's just in the eye socket, so far as I know. And... I can't be sure, but I wouldn't be inclined to assume that; the ability to see mushi can be brought on by encounters with them, but some people just seem to be born with the ability."