Maybe..... PEIN IS OBITO!!!!!!!!

Seriously that would suck, but I think it may even be the case that Pein himself could be some frail character who has lost his own personal ability to fight since he is so used to using other bodies to do his bidding.

If that's the case he could like anything and anyone, and thus be anywhere... which would make him difficult to find.

Another thought is about the Rinnengan... it always seems to be on, and you never see Pein without it. Now I think that either he himself has a large amoutn of chakra to always have it on, or he is powering it using a Bijuu. If it is the latter it could mean that he is close to that statue where all the Bijuu's are trapped.