the No-One-Must-Know obsession — "Hal has no idea why this…"
Hal has no idea why this is, or whence, this obsession with the secrecy of it. He broods on it abstractly sometimes, when high: this No-One-Must-Know thing. It’s not fear per se, fear of discovery.
Context
The compulsion inside the compulsion, and the fact that Hal cannot locate its source. It is not simple fear of getting caught — the cadre already knows he uses; what he cannot let anyone see is the state itself, the being-high, and the obsession is described as almost irresistible in its force, stronger than the wish to get high in the first place. He broods on it and the brooding leads nowhere, which is exactly the trouble.