I think that they may go to "another place"possibly with twolegs. Some kittypets (like pinestar) go to starclan. Maybe even princess , since she gave cloudtail to the clans and knew of starclan somewhat. I feel like maybe all of the rouges and loners go all together maybe in another place. Also , sometimes i feel they can possibly pass through if they believe in starclan. This is just my thoughts though.