Are you talking about the kitten they found in chapter 4? Because I think that's a different cat.
Chapter 4 kitten: Miko and Hana found it in a box and we're like, "Did someone leave it here," which sorta implies that it was too young to get out of the box.
Shiro: the shop lady said that Shiro used to come and curl up in front of the store and that hearing about him "took her back." So a cat that was old enough to roam around and probably died a while ago.
if the kitten has his eyes open, he can move, 3month cat can move without problem in and out of a box, and cat love carton box to rest, moreover, they have the same fur color.
plus miko and friend assume that the kitten was abandonned, thank to the lack of collar, but it can be just a domestic cat that love to wander, that happen with big adult cat that are notoriously know to wander to get food from multiple family.
