What clue?
Here is a thought.

Suppose I left something unmistakably Chinese?
Tarling had evidently been friendly with the girl ...

something Chinese might place him under suspicion...." The diary ended with the word "suspicion," an appropriate ending.

Tarling read the passages again and again until he almost had them by heart.

Then he closed the book and locked it away in his drawer.
He sat with his chin on his hand for half an hour.