there's that option as well, maybe Sasori is hiding somewhere and the body we're seeing now is just the puppet, like how Kankaru was doing first time we saw him...
though that since Deidara is there talking, I'm guessing that the real body is at least there (inside the puppet body, or whatever).
then again, there's the possibilty that sasori is using more than one doll (again, Kankaru case), perhaps the 300 human doll he was talking about is a diffrent one that he was working on for a while, and will soon be complete?