OOC The official journal - the one which contains the evidence he distills into his reports to the SS - is hidden in an obvious place: wrapped and taped up in four layers of cellophane, weighted and sunk in the tank of the toilet in his official residence, the one known to the Germans. It is written in coded German, and contains nothing that is not already known to his employers.
The unofficial journal, which he really shouldn't keep and he knows this, in which he keeps track of contacts and actions and plots his movements against those of the Resistance and the Nazis like some sort of three-way chess game - this is in coded Occitan, hidden inside the wall of the apartment he uses when he plays at being French. Also hidden in this apartment, above the ceiling, is a radio he uses to pick up transmissions from Britain and other places in the world.
A third journal, in a third code, also in Occitan, is where he records his private thoughts, on the rather rare occasion that he has any he feels he must express. This is hidden inside of a floorboard (they are very thick - of ancient construction - and he split it in order to hollow part of it out) beneath the bed in his most private residence, a small room whose existence he has shared with no-one.