Pica uses the Stone Stone fruit, which in recent episodes, shows him
turning into a giant stone colossus.
But, what happens to his human body when he uses his fruit in this way?
Well, in the most recent episode you saw
Ruffy destroying the head of the big stone giant Pica created, while Zorro states, that the stone body is just a hull and that Pica can recreate it. Only seconds later you see Pica coming out of the stone body itself, so I guess it is not wrong to say: He hides somwhere insided the stone body he created.