Gambling.

• No object. There's no baggie, bottle or pipe, nothing to possess, so nothing to find, seize or throw out. The "substance" is just numbers on a screen.
• It goes straight to your pocket. The damage is financial from day one, and it takes savings, then the house, then other people's money.
• No physical signs. No smell, no pupils, no weight loss, so it can stay hidden for years.
• It's perversely not about winning. The hook is the anticipation and the chase, so a win doesn't satisfy and a loss doesn't stop it.

It also has one of the highest suicide rates of any addiction, partly because by the time it's discovered, everything is already gone.