In the beginning Kakashi says that he lost everyone like Sasuke.
It wouldn't make sense if he was just comparing a few people to the slaughtering of a whole clan.
With that it only makes sense if Kakashi is a senju.
Only known living Senju is Tsunade
I think they are all over the world, married within other clans, because they don't have an unique power. Shodaime was only Mokuton user.