the secrecy about the secrecy
the odor of obsession about the whole thing. The secrecy about the notebook, and the secrecy about the secrecy.
Context
The tell that turns habit into disease is doubled concealment: not just the hidden notebook but the hiddenness of the hiding, the furtiveness about the furtiveness that no one can quite point to as evidence of anything wrong. It is the exact structure the book gives Hal's pump-room attachment — the secrecy itself become the load-bearing thing, more diagnostic than the object it guards, an odor of obsession you can smell before you can name.